The analysis of heat pump cycles with and without an internal heat exchanger (IHE) is carried out in the paper, in which HFC125/HCs binary mixtures are used as the alternative refrigerants. And the cycle performance under different operation conditions is also compared. The results show that when the mass fraction of HFC125 ranges from 10 to 20%, the coefficient of performance (COP) for HFC125/HC290 (M1) mixtures is 0.92 and 1.01% lower than that of HCFC22 and HFC134a, respectively. For HFC125/HC600 (M2) and HFC125/HC600a (M3) mixtures, the COPs are higher than those of HCFC22 at the mass fraction of HFC125 between 0 and 74.1%, 0 and 66.5% in the mixtures, respectively, and compared with HFC134a, the COPs and volumetric heating capacities are higher when the mass fraction of HFC125 is between 38.6 and 73.3%, and 30.8 and 66%, respectively. For HFC125/HC1270 (M4) mixtures, the COPs are always lower than those of HCFC22 and HFC134a. It is also found that the IHE has a slight effect on the COPs with varying the mass fraction of HFC125 in the binary mixtures. The results obtained can provide some useful guidelines for the choice of alternative refrigerants. Copyright (c) 2011 John Wiley & Sons, Ltd.