The aim of this work is to formulate a mathematical model of water vaporization at a moving front in porous media, taking into account capillary forces. The complete system of boundary conditions at the boiling front is presented. A similarity analytical solution is then derived for the one-dimensional non-linear problem of mass extraction from a contact boundary between a hydrothermal reservoir and impermeable rocks. In this case the problem leads to a system of transcendental equations that is solved numerically. The model provides qualitative and quantitative information on the influence of capillary forces on the water vaporization process in geothermal reservoirs. It is shown that two different regimes of water vaporization exist: (1) with formation of a two-phase transition zone and (2) with formation of a sharp vaporization front. An increase in capillary forces in water wettable media leads to the extension of the domain of the frontal regime and to an intensification of the vaporization process; vice versa in non-wettable media. (C) 2003 CNR. Published by Elsevier Science Ltd. All rights reserved.