The coefficients of diffusion for water vapor in snow and other porous media are fundamental parameters of great importance. While they are generally thought to be greater than the coefficient for air, there is much uncertainty about what value to use. Attempts to determine the diffusion coefficient in snow experimentally have led to contradictory results, but Yosida's experiments seem to give reasonable results. Yosida's idea of hand-to-hand diffusion is quantified here in a particle-to-particle model which shows that the diffusion coefficient depends on the mean pore size and that the coefficient in snow is four to seven times greater than in air.