R22 being a HCFC is to be phased out soon as per the Montreal protocol. This paper presents the experimental performance study of an air-conditioner retrofitted with M20 (a refrigerant mixture of R407C, R600a and R290 in the mass fraction 80:10.96:9.04 respectively). M20 is considered as a substitute for R22 in window air-conditioners with mineral oil as the compressor lubricant. Previously, the performance of the same mixture has been reported with 19% extra condenser length as compared to that in the R22 system. In this study, experiments were conducted in a 5.25kW window airconditioner to assess the potential of M20 as a retrofit refrigerant without making any system alteration other than changing the capillary. The system performance study was carried out with R22 and later with the M20 charge in a psychrometric test facility as per BIS and ASHRAE standards. Experimental results showed that with the M20, the system had 4.18% to 7.47% lower cooling capacity with respect to that of R22. The COP for the M20 refrigerant was lower, in the range of 0.58% to 7.78% than that of R22. Hence, M20 can be used as a drop-in-substitute for R22 in window air-conditioners but with a marginal reduction in cooling capacity.