Water as a refrigerant (R718) is compared with some current natural (R717 and R290) and synthetic refrigerants (R134a, R12, R22, and R152a) regarding environmental issues including ozone depletion potential (ODP) and global warming potential (GWP), safety (toxicity and flammability), operating cost, refrigeration capacity and coefficient of performance (COP). A computer code simulating a simple vapour compression cycle was developed to calculate COPS, pressure ratios, outlet temperatures of the refrigerants from the compressor, and evaporator temperatures above which water theoretically yields better COPs than the other refrigerants investigated. The main difference of this study from other similar studies is that both evaporator temperature and condenser temperature are changed as changing parameters, but the temperature lift, which is the temperature difference between condenser and evaporator, are held constant and the irreversibility during the compression process is also taken into consideration by taking the isentropic efficiency different from 100%. It is found that for evaporator temperatures above 20 degrees C and small temperature lift (5 K), R718 gives the highest COP assuming exactly the same cycle parameters. For medium temperature lifts (20-25 K), this evaporator temperature is above 35 degrees C, whereas for even greater temperature lifts it decreases again. Furthermore, with increased values of polytropic efficiency, R718 can maintain higher COPS over other refrigerants, at lower evaporator temperatures. Copyright (c) 2005 John Wiley & Sons. Ltd.