This work presents a new method to evaluate the effective diffusivity in porous solids, considering the mass-transfer mechanisms occurring in the pores and on the surface. The binary friction model was used to evaluate the effective gas-phase diffusivity and the Maxwell-Stefan surface diffusion equation to the transport of bound water. Different expressions for the effective diffusivity for porous solids were derived considering parallel diffusion, series diffusion, and the Clausius-Mossotti model. The numerical results were compared to experimental data obtained for Son clay. In the pore diffusivity, when both are present, capillary flow showed dominance over gas diffusion. For the entire moisture content range, only a combination of the models presented good agreement with experimental data, showing changes in the interdependence of mechanisms during the drying process.