A study of the thermodynamic performance of nonazeotropic refrigerant mixtures (NARMs) involving R-143a/R-134a was completed. The aim of the study was to identify possible replacements for R-12. Two refrigeration cycles were considered, the vapor-compression cycle and the internal heat exchanger cycle. Our results suggest that the nonazeotropic: R-143a/R-134a mixtures studied yield comparable performance to R-12 when the internal heat exchanger cycle is used.