The refrigeration cycle of a reversible water-to-water heat pump has been investigated at different operation parameters of a laboratory installation. The investigation has been accomplished in cooling working conditions. The real operation cycles of the pump have been plotted in “logp-h” diagram by determination of definite thermodynamic parameters of the refrigerant. For this purpose, COOL PACK software has been used. The investigated refrigeration cycles are characterized by different switch off temperature of the heat pump. Based on the obtained results, the following parameters characterizing the heat pump’s efficiency have been determined: specific heat of vaporization, specific compression work, specific heat of condensation, and coefficient of performance.