Abstract: Gas diffusion in a microporous media is strongly analogous to light transport in a scene. Interaction of light with surfaces has a counterpart in Knudsen diffusion and light dispersion in participating media is very similar to gaseous bulk diffusion. On behalf of this fact, a Monte-Carlo simulation method has been designed to compute gas diffusivities and related properties in 3D images of porous media, making use of techniques originated in the domain of image rendering, such as tesselation, spatial partitioning, distance images, optimized intersection tests as well as in the domain of physical simulations, such as microscopic/macroscopic random walk algorithms. Putting all these methods together results in a very efficient algorithm allowing to determine accurately transport properties in real porous media from imaging techniques such as X-ray tomography.