In order to improve high temperature heat pumps (HTHP), firstly, after theoretical analysis is conducted about the heat transfer of non-azeotropic working fluids in the condenser, the possibility of incomplete condensation of non-azeotropic working fluids in the condenser of a HTHP is presented. Secondly, an experimental plan is designed to study the phenomenon, and many working conditions experiments (flow rate, temperature and frequency are changed) are done on a water-to-water heat pump unit. Finally, the incomplete condensation is found, and some relationships are presented between the phenomenon and the parameters, which are helpful to improve the control strategy of the HTHP. (c) 2005 Elsevier Ltd. All rights reserved.