An analytical study is performed to demonstrate the effect of viscous dissipation on the performance of a counter-how recuperator. Four liquids, namely, water, transformer oil, engine oil, and glycerol are selected as working fluids. It is shown that a considerable decrease in performance is obtained when the viscous frictional heating is considered. The optimum size of recuperator is found to be finite and depends on the thermal properties of the running fluids. The effect of viscous heating on the temperature variation in the recuperator is also discussed. (C) 1998 John Wiley & Sons Ltd.