A water-to-water high temperature heat pump was studied experimentally. The performance of the system was characterized by refrigerant compositions, compressor RPM and water temperature change. For the experimental conditions of the inlet water temperature of evaporator of 40 degreesC and the inlet and outlet water temperatures of the condenser of 70 and 80 degreesC, respectively, the experiment shows that the coefficient of performance is maximum when the molar component of R22 is about 75%. It is shown that the maximum pressure of the system is under 2.5 MPa after taking R22/R141b as working fluids, even though the highest cooling water temperatures is about 80 degreesC.