An experimental study of heat and mass transfer in natural and forced convection in a vertical channel with parallel plates is presented. The channel is designed to play the role of an evaporator. The vertical walls are formed by two metal plates having a good thermal conductivity. These two plates are symmetrically heated and subjected to a constant heat flux. On the inner face of each wall, a thin film of a negligible thickness falls. Submitted to a heat flux, this liquid partly evaporates. The experimentation consists in determining the flux of evaporation of the falling liquid film in natural and forced convection, The exploration of the thermal field (in the film and in the gas phase) allows us to give a description of evaporation in the channel. The results obtained are exploited to study the influence of the operating parameters such as the heat flux density on the temperature of liquid film and the evaporated flux for the two modes. (C) 2009 Elsevier B. V. All rights reserved.