The diffusion of moisture in soybean grains is usually followed by two physical facts related to the presence of water: the increase in grain size due to water accumulation and the increase in permeability, as more water is absorbed. In order to increase the physical meaning of the proposed model those two phenomenological facts were inserted in the classical Fick's Second Law of Diffusion. The increase of the grains was taken into account by considering the radius of the grains as a moving boundary of the diffusion system (characterizing a Stefan problem). The increase in permeability was taken into account by considering an exponential dependence of the diffusivity with the moisture content. The behaviour of the main parameters was analyzed with the temperatures. The moisture profiles were calculated for the moving boundary problem as well as the behaviour of the radius of the grains as a function of time. The results showed good agreement with experimental reality of soybean hydration process.