The batch reaction time profile of the reactive extraction of biodiesel from rapeseed particles was modelled by postulating that the reaction occurs inside the seed and the products diffuse to the bulk solvent. The model indicates that at high catalyst concentration, the overall reactive extraction rate is controlled by the internal diffusion rate. This means that the rate of the process can be improved if the diffusion coefficient in the seed can be increased, i.e. by means of seed pre-treatment.