R503 Refrigerant Reference
Updated October 12, 2026
R-503
Historical azeotrope of R-23 (HFC) and R-13 (CFC), 40.1/59.9 by mass. Normal boiling −126°F — one of the lowest of any commercial refrigerant. ASHRAE A1. ODP 0.599, GWP 14,560. Banned for production in 1996 under the Montreal Protocol because the R-13 component is a CFC. Used historically as cascade low-stage refrigerant for environmental test chambers.
Not yet checked against an independent source.
Lower toxicity (Occupational Exposure Limit ≥ 400 ppm). No flame propagation in air at standard atmospheric pressure and 60°C. R-134a, R-22, R-410A, R-404A, R-744 (CO2) are A1.
- Flammability
- None (no flame propagation)
- Toxicity
- Lower (OEL ≥ 400 ppm)
Classification per ANSI/ASHRAE Standard 34-2022. See full reference.
Saturation pressure-temperature curve
Retired refrigerant — no current commercial PT chart. R-503 is no longer in commercial service (Montreal Protocol / regional phaseouts). Historical PT data exists in archived ASHRAE Handbook editions but is not maintained in this site's current dataset. This page remains for historical and identification reference.
- historical-referenceASHRAE Handbook of Refrigeration (historical editions, pre-2010) — archived PT tables for R-503 (R-23/R-13 azeotropic CFC blend).Reference historical data only. R-503 contained R-13 (CFC) and was phased out under the Montreal Protocol; no production since 1996.https://www.ashrae.org
- regulatoryMontreal Protocol on Substances that Deplete the Ozone Layer (1987, amended).https://ozone.unep.org/treaties/montreal-protocol
At a glance
Chemistry
Lubricant compatibility
Azeotrope. Historically used as the low-stage refrigerant in cascade systems for environmental test chambers and cryogenic applications. Banned for production.
Blend composition
- R-23 40.1%
- R-13 59.9%
Common applications
- Cascade low-stage refrigeration (legacy)
- Cryogenic and ultra-low-temperature systems (legacy)
Properties
- Boiling point (1 atm)−126.0°F (−87.8°C)
- Critical point67.1°F at 612 PSIG
- Molar mass87.30 g/mol
- Temperature glideNegligible (0.00°F)
- ODP0.599
- GWP (100-yr)14,560 US EPA value, 40 CFR 84.64(b) IPCC: AR4 14,560 · AR5 13,299 · AR6 15,558
- New-equipment status (US)Class I ODS (CFC) — US production/import ended January 1, 1996.
What is R-503?
R-503 is an azeotropic blend of R-23 (trifluoromethane, HFC) and R-13 (chlorotrifluoromethane, CFC) in 40.1/59.9 mass ratio. The combination produces an azeotrope with normal boiling point of −126°F (−88°C) — exceptional even by ultra-low-temperature refrigerant standards. Critical temperature is only 67°F, severely limiting the operating range to deeply sub-ambient applications.
R-503 was used as a cascade low-stage refrigerant for environmental test chambers and cryogenic systems. The azeotropic behavior provided service simplicity (no glide management) while the very-low boiling point matched the requirements of −100°F to −200°F application ranges that single-stage refrigeration cannot reach.
The R-13 component is a CFC — production was banned in the US on January 1, 1996 under the Montreal Protocol. R-503, as a CFC-containing blend, was banned alongside R-13.
Where R-503 is used
- Cascade low-stage refrigerant for environmental test chambers (legacy, pre-1996)
- Cryogenic refrigeration for scientific and industrial applications (legacy)
- Specialty refrigeration in aerospace component testing (legacy)
- No new equipment since 1996; reclaimed supply is essentially exhausted
Regulatory & phase-down status
R-503 production has been banned for almost three decades. The 640-year atmospheric lifetime of the R-13 component means molecules released decades ago are still in the stratosphere actively depleting ozone. The R-23 component (atmospheric lifetime 222 years, GWP 14,800) adds significant climate impact independent of ozone effects.
For remaining R-503 equipment encountered today, the practical path is full replacement with modern cascade systems using R-23 alone as the low-stage (HFC, no ODP but still high GWP) or R-744 (CO₂, GWP 1) for less-demanding low-temperature applications. Autocascade systems with mixed-refrigerant blends are another modern approach.
Service notes
Mineral oil (MO) is compatible. POE is not. EPA Section 608 Type II certification covers R-503. Recovery is required, though in practice R-503 service is virtually nonexistent given the equipment has aged out.
The azeotropic behavior (zero glide) was a service advantage compared to non-azeotropic blends — R-503 was handled like a pure refrigerant for saturation measurements.
Phase-down timeline
R-503 contains HFC-23, a regulated substance under the AIM Act. Its 40 CFR 84.64 GWP is 5,935. 84.54's industrial-process categories cover −50 °C (−58 °F) and warmer; R-503's ultra-low-temperature cascade use is below that, so no 84.54 category lists it.
HFC production & consumption allowances step down to 30% of baseline in 2029–2033, 20% of baseline in 2034–2035, and 15% of baseline in 2036 and later (42 U.S.C. 7675(e)(2)(C)).
R-503 is also an ozone-depleting substance controlled under the US Clean Air Act (class I — CFC). Milestones below are US production/import limits.
- January 1, 1996 — CFC-13: US ban on production and import took effect[1].
US new-equipment rules (40 CFR 84.54)
R-503 contains HFC-23, a regulated substance under the AIM Act. Under 40 CFR 84.64(b) its GWP is 5,935: each component's GWP times its mass fraction; components without a GWP under 84.64 are left out (84.64(c)).
84.54's industrial-process categories cover −50 °C (−58 °F) and warmer; R-503's ultra-low-temperature cascade use is below that, so no 84.54 category lists it.
Servicing. 40 CFR 84.54 restricts new manufacture/import, later sale/distribution, and installation; repairing or servicing existing R-503 equipment is not restricted by 84.54.
Source: 40 CFR 84.54, current through 91 FR 31329 (May 26, 2026). Installation defined at 40 CFR 84.54(e).
Global warming potential, in context
Retrofit and replacement paths
Replacements for R-503
Frequently asked
›What was R-503 used for?
Cascade low-stage refrigeration in environmental test chambers and cryogenic systems requiring evaporator temperatures of −100°F to −200°F. The azeotropic R-23/R-13 blend gave service simplicity (no glide) combined with the very-low boiling point needed for these applications.
›Why was R-503 banned?
The R-13 component (59.9% of R-503) is a chlorofluorocarbon. R-13 has ODP 1.0 — one of the highest of any commercial refrigerant. The Montreal Protocol mandated CFC phase-out; US production was banned on January 1, 1996. R-503, as a blend containing R-13, was banned simultaneously. The R-23 component is HFC (zero ODP) and would not have been banned on ozone grounds, but is now itself subject to AIM Act phase-down for its very high GWP (14,800).
›What replaced R-503?
For new cascade low-temperature systems: R-23 alone (HFC, zero ODP, high GWP 14,800), R-744 (CO₂, GWP 1, but limited to applications above CO₂'s triple-point temperature of −70°F), or autocascade systems with mixed-refrigerant blends optimized for specific temperature ranges. R-23 is the closest direct substitute in terms of physical properties but has its own GWP concerns; the trajectory is toward CO₂ where applicable and toward novel cascade designs for very-low-temperature applications.
›Is R-503 an azeotrope or zeotrope?
Azeotrope. The 40.1/59.9 R-23/R-13 composition was chosen specifically to produce azeotropic behavior at typical operating pressures. Bubble and dew points coincide; service measurement of superheat and subcooling can be done without distinguishing bubble vs dew curves. This was a major service advantage compared to non-azeotropic blends.
›What is the GWP and ODP of R-503?
GWP 14,560, mass-weighted from R-23 (14,800) at 40.1% and R-13 (14,400) at 59.9%. Both components have high GWP — the blend ranks among the highest-GWP commercial refrigerants. ODP 0.599, from the R-13 component (R-13 has ODP 1.0, mass-weighted to 0.599 in the blend). The combination of high ODP and very-high GWP made R-503 environmentally unviable from the moment the Montreal Protocol was negotiated.
Sources & citations
- [1]Phaseout of Class I Ozone-Depleting Substances. US EPA. last updated March 10, 2026. https://www.epa.gov/ods-phaseout/phaseout-class-i-ozone-depleting-substances (accessed October 5, 2026)
- [2]2017 Guideline for Assignment of Refrigerant Container Colors (AHRI Guideline N-2017). AHRI. 2017. https://www.ahrinet.org/system/files/2023-06/AHRI_Guideline_N_2017.pdf (accessed October 2, 2026)