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 13,600. 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.
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-503is 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-2340.1%
- R-1359.9%
Common applications
- Cascade low-stage refrigeration (legacy)
- Cryogenic and ultra-low-temperature systems (legacy)
Properties
- Boiling point (1 atm)-87.8°C / -126.0°F
- Critical point67.1°F at 612 PSIG
- Molar mass87.30 g/mol
- Temperature glideNegligible (0.00°F)
- ODP0.599
- GWP (AR5, 100-yr)13600
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 the dominant cascade low-stage refrigerant for environmental test chambers and cryogenic systems from the 1960s through the early 1990s. 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 (rare), 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
Global warming potential, in context
No peer-comparison group is defined for R-503. Its 100-year GWP per IPCC AR5 is 13600 — above the EPA AIM Act 700 GWP cap and well above the EU F-Gas 150 cap.
Peer-comparison groups are defined for refrigerants that compete in the same application sector (residential AC, commercial MT/LT, chillers, mobile AC). Specialty or research-grade refrigerants without a clear peer set don't appear in any group; their GWP is shown above in absolute terms instead.
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. Common end-uses included aerospace component qualification testing, semiconductor materials characterization, biomedical research, and pharmaceutical manufacturing.
›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 EPA 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 13,600 per IPCC AR5, 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.