The study addresses problems of moving boundaries that involve mass diffusion in a spherical swelling polymer-penetrant system with a controlled release of drug. The method of perturbation is applied and the complete time history of the penetrant front and the volume expansion front are determined for small control values of e; the local similarity method is used to expand the results for large e. The results reveal that the velocity of penetrant and the release rate of the drug near t* = 0 are constant, regardless of the diameter of the sphere. The velocity and rate decrease over a short period, and then increase again as the penetrant front proceeds to the sphere center. If epsilon and (v) over bar are small, the volume expansion front is almost unchanged and only the time history of the penetrant front in the swelling polymer-penetrant system needs to be considered. (C) 2004 Elsevier Ltd. All rights reserved.