An experimental study on the capacity control of a heat pump system has been performed using refrigerant mixtures of R32/134a. A test apparatus was made of a refrigeration part and two different types of composition changing parts; a single separator system and a separator-rectifier combined system. Analysis of the separation process was made for a basic single separator system. In order to pursue a wider range of composition change, a separator-rectifier combined system with packed-type distillation column was designed with Raschig ring as packing material. The composition changing part was connected to the condenser outlet and the evaporator inlet. Heating capacity, cooling capacity, and coefficient of performance (COP) of the system were measured under heating and cooling conditions. When the single separator system was used as a composition changing part, the range of composition change in the refrigeration system was approximately 13%. Around 26% of composition change was obtained using the separator-rectifier combined system. From the composition change with the separator-rectifier combined system, the capacity improved from 2.6 to 3.4 kW in the cooling test and from 1.8 to 2.4 kW in the heating test. As the composition of R32 increases, heating and cooling capacities were improved, whereas the value of COP with the refrigerant mixture is enhanced due to a temperature glide effect. It is concluded that the system capacity can be adjusted to meet load requirements by controlling the composition of the refrigerant mixture. (C) 2003 Published by Elsevier Ltd.