The performance of a cross-counterflow, gas-liquid heat exchanger was studied experimentally with a full-scale test apparatus. The gas-side heat transfer surface was equipped with needle-shaped fins. The main purpose of the research was to define the heat transfer efficiency, as well as the external and internal pressure drops and the effect of the fouling on the thermal-hydraulic characteristics of the heat exchanger. The Reynolds number of moist air in the channel, before the heat exchanger, varied between 60 000 and 150 000. Four different liquid mixtures were used as a heat transfer liquid. The velocity of liquid inside the tubes was set between 0.5 and 1.25 m s(-1). Then the Reynolds number varied from 500 to 1500 with propylene glycol-water mixture, from 3000 to 8000 with potassium formiate-water mixtures and from 5000 to 30 000 with water. The finned outside surface of the heat exchanger was fouled by two kinds of test dust; pure quartz and standard test dust. Copyright (C) 2003 John Wiley Sons, Ltd.