In this paper, a model of the isothermal chemical vapor infiltration (ICVI) is developed. The fibrous substrate (preform) is considered as a complex porous medium with two systems of parallel pores oriented along the preform thickness. One of them is associated with pores formed by fibers in a bundle; another is a system of inter-bundle pores. The model accounts for the convective processes governed by phase transitions and transversal mass exchange between the pore systems. The computations are performed for the case of the production of SiC-matrix composite. The effect of the convection and the mass exchange between the pore systems on the infiltration process is studied. The mass exchange is found to homogenize the vapor composition in different pore systems. The analysis reveals that the above phenomena govern significantly the infiltration time and residual porosity especially in the pores within the bundle. (C) 2004 Elsevier B.V. All rights reserved.