The performance of the heat pump was predicted using a fuzzy-logic controller under various working-conditions and mixing ratios of R12/R22 refrigerant mixtures, instead of requiring an expensive and time consuming experimental study [Int. J. Ref. 13 (1990) 163]. Fuzzy-logic's linguistic terms provide a feasible method for defining the performance of the heat pump. Input data for the fuzzy logic are mixing ratio, evaporator-inlet temperature and condenser pressure. In the comparison of performance, results obtained via analytic equations and by means of the fuzzy-logic controller, the coefficient of performance (COP), and rational efficiency (RE) for all working situations differ by less than 1.5% and 1%, respectively. The statistical coefficient of multiple determinations (R-2-value) equals to 0.9988 for both the COP and the RE. With these results, we believe that fuzzy logic can be used for the accurate prediction of the COP and the RE of the heat pump. (C) 2004 Elsevier Ltd. All rights reserved.