The object of the present work is one of the new devices used to deliver the drug at a controlled release rate: microparticles made up of a solid drug core surrounded by a polymer coating. A theoretical model has been developed to describe the release of the microencapsulated solid active principle: the diffusion process has been modelled in detail and the temporal evolution of the drug concentration profile has been evaluated in the polymer shell and in the encapsulated solution. Then a sensitivity analysis has been carried out to investigate the influence of certain system characteristics and operating conditions on the lag time, on the core dissolution time and on the release curve. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.