Theoretical and experimental researches are investigated about the method of determining die proportions of the components of zeotropic mixtures presented by Venkatarathnam, and theoretical analysis and computation are performed on the zeotropic mixtures W1 (wt: R22/R142b, 0.510.5) and W2 (wt: R22/R142b, 0.26/0.74). From the results, the COP of W2 is appreciably better than that of W1, but W2 has a non-linear variation of the two-phase enthalpy and temperature during phase change. In diversified experimental working conditions, the COP values of W2 get close to those of W1, as well as the values of their thermodynamic perfections. Therefore, it is necessary to analyze the proportions of the components of zeotropic mixtures, especially to avoid reducing the selection range of working fluids just because of a little improvement of efficiency and increasing other costs such as those affecting economical efficiency. (C) 2007 Elsevier Ltd. All rights reserved.