A plate heat exchanger is tested under two-phase flow conditions by using an air/water mixture as the cold stream. Visual observations recorded by a high-speed video camera lead to the construction of a flow regime map. The heat transfer coefficient of the air/water stream is measured as a function of air and water superficial velocities. The flow regime with a gas-continuous phase covering the core of the channel and liquid flowing in the form of rivulets inside the furrows shows particularly favorable heat transfer characteristics. (C) 2002 Elsevier Science Ltd. All rights reserved.