Oil migration through filled chocolate products during storage periods is inevitable and is responsible for the loss of the original quality of the product. A model that predicts the extent of oil migration can help food engineers optimize the post-production period, thereby delivering an acceptable quality product to consumers. In this work, a predictive model is proposed based on an explicit formulation of the diffusion problem in terms of the molecular diffusivity and the internal microstructure of chocolate. This work overcomes the limitations of the previous models by including a moving boundary, which allows for swelling of the chocolate slab and replacing the effective diffusion coefficient with a more physically meaningful one, which includes a tortuosity term that varies with oil concentration. The model has been validated against experimental mass uptake curves. (C) 2008 Elsevier Ltd. All rights reserved.