A pore-scale model is developed to simulate mass transfer between two fluid phases in a porous medium. The approach uses a network description of the pore space and builds on explicit tracking of the fluid-fluid interfaces in the pore network. Mass transfer is computed as local mass fluxes across each interface, and transport equations are solved in the pore network by a characteristic method. The concept of stagnant-layer diffusion is used to describe the interface mass transfer, where calculated local concentrations control the rates of mass transfer. The model results predict dissolution fronts developed in column experiments of porous media initially at residual nonaqueous phase saturation. The definition of macroscopic mass transfer coefficients is investigated, and comparisons are made for rigorously upscaled quantities. ¿ 2001 American Geophysical Union