Mass or heat transfer may cause an increase or a decrease in the size of the analyzed system. Moving boundaries are present in phase exchange processes (heat) and in moisture diffusion in polymers and grains (mass), for example. In the case of grain hydration, the increase in grain size can reach 30-40% of the initial grain size. Fick's Second Law of Diffusion was used for modeling conventional and transgenic soybean grain cultivars. The model was modified to consider the increase in the spatial grid caused by the increase in grain size during hydration. The moisture profiles were obtained for both cultivars and then validated using experimental data on moisture as a function of time for five different temperatures, fitting very well to said experimental data. Grain radius as a function of time, as calculated by the model, was also compared with experimental data. There was a slight difference between the values for the radius calculated by the model and those from experimental data due to the, disproportionate growth of the soybean seedcoat in a preferred direction. The proposed model has been satisfactory for the mathematical description of grain hydration with increase in grain size. Its applicability may be extended to spherical systems undergoing changes in radial direction during mass diffusion processes. (C) 2015 Elsevier Ltd. All rights reserved.