Most of the present formulations for the mass conservation of species do not correctly represent changes in the time scale of diffusion as a function of porosity in a system of porous media. In sediments, or in any porous system, the presence of solid particles causes the diffusion paths of species to deviate from straight lines. To represent the role of porosity on diffusion, the diffusion coefficient must be scaled with tortuosity. In this paper we present a review of the available formulations for the scaled diffusion coefficient with tortuosity, sensitivity tests with analytical solutions, and calibration of these formulations with respect to measured data. (c) 2007 Elsevier Ltd. All rights reserved.