This paper presents results from an experimental and theoretical investigation of diffusion of methane gas in three types of porous media: soil, compost and soil-compost mixtures. The experimentally determined diffusion coefficient data showed that moisture content is an important parameter that controls methane gas diffusion in compost rich granular media. The experimentally determined diffusion coefficient values were compared with values calculated from empirical models currently available in literature. This analysis showed a number of weakness in current models; including the current models under-predicted diffusion coefficient at high granular media porosities and over-predicted at low granular media porosities. The experimental results were used to develop a new empirical model to estimate the diffusion coefficient of granular material with high compost content. The new empirical model consists of four fitting parameters and is more robust and flexible than the currently available models. (C) 2011 Elsevier B.V. All rights reserved.