The landscape of fluids used inrefrigeration and air conditioning is changing rapidly. Fluids that have somepotential to deplete the ozone layer are on the way to being banned. Butconcerns are also raised about the high global warming potential (GWP) of someof their potential alternatives. This article discusses a methodology tocompare the properties of various fluids with a wide range in properties,including zeotropic blends, based on theoretical cycle calculations.