A ternary mixture of R124/R142b/R600a, named HTR01, for moderately high temperature heat pumps, was developed. Tests of material compatibility and oil miscibility showed that the mixture could be used with a R22 compressor in an HTR01 heat-pump system. A 2.92 kW moderately high temperature water source heat pump system was set up with HTR01 as the refrigerant to study the system performance with HTR01. Then, a 300 kW moderately high temperature water source heat pump system was built with HTR01 as the refrigerant to test the performance with a geothermal hot water source. The test showed that the condenser outlet water temperature could reach and hold on about 90 degrees C with a high coefficient of performance. (c) 2004 Published by Elsevier Ltd.