A comparison of many organic working fluids was made. Emphasis was given to the efficiency of the thermodynamic cycle using each working fluid, as well as its environmental impact. The thermodynamic cycle consists of two stages: high temperature and low temperature. The maximum temperature of the first one is around 137 degrees C and the second one 77 degrees C. Not all the working fluids had a critical temperature higher than 137 degrees C; therefore, not all could be used under these conditions. The study focused on 33 organic working fluids; according to various criteria that were followed, the most appropriate found was the R245fa. Although it was not the most efficient, nor produced the maximum mechanical work, it performed quite well and, in addition, its use is not restricted by any international regulations. The concept was part ofthe research within the framework ofthe project COOP-CT-2003-507997.