In the present paper a 3D non-stationary two-sided mathematical model of joint motion of evaporating liquid film and cocurrent gas flow in a microchannel with local heating has been developed. This model takes into account a deformable gas-liquid interface, convective heat transfer in the liquid and the gas phases as well as temperature dependence of surface tension and liquid viscosity. Assuming the lubrication theory to be valid, the problem has been reduced to five governing equations for the film thickness, temperature fields in the gas and liquid, vapor concentration in the gas phase and gas pressure. Numerically it is shown that for films sheared by gas in microchannels vapor is transported by forced convection and diffusion, and diffusion plays the most considerable role in vapor transport at low gas velocities. Also, it is shown that concentration and thermal boundary layers are formed. The boundary layers have a specific S-shaped form. The width of the vapor track increases along the gas flow direction in front of the heater and decreases downstream the heater. The distance over which the width decreases is an order of magnitude higher than the heater length. This fact can be explained by the condensation of the vapor. (C) 2013 Elsevier Ltd. All rights reserved.