[en] It is attempted to define more environment friendly working fluids for the heat pump of ground source heat pump systems (GSHPSs). The examined case is an office building with total cooled area 1000 m"2, which is connected with such a system. The energy demands are computed by TRNSYS 16.1 and the ground loop is modeled by GLD2009. Alternative refrigerants are proposed instead of the one that has been extensively used worldwide, for many years, the R-22. The proposed replacements are the binary mixtures R-32/R-134a in compositions 20/80%, 30/70%, 40/60% by mass, and the ternary blends R-407B (10wt% R-32, 70wt% R-125, 20wt% R-134a), R-152a/R-125/R-32 (48wt% R-152a, 18wt% R-125, 34wt% R-32), R-410B (45wt% R-32, 55wt% R-125) and R-507A (50wt% R-125, 50wt% R-143a). A MATLAB code is developed in order to calculate coefficient of performance (COP) values for one stage vapor compression reference cycle with no evaporator superheating and condenser subcooling. The thermodynamic properties of refrigerants are derived from previous works of our laboratory, REFPROP 8.0 and international bibliography. The choice of the most suitable refrigerant depends on the criteria which are set.