Ethylcellulose microcapsules containing theophylline were prepared by the phase-separation coacervation process induced by non-solvent addition in the presence of variable amounts of polyisobutylene. No statistical difference was evident between the different sized microcapsules with respect to their drug content, wall thickness and in-vitro drug dissolution profiles. Poly-isobutylene was, however, found to influence the release profiles of drug to a great extent. An increase in polyisobutylene concentration retarded the release of theophylline from the microcapsules due to the formation of smaller and spheroidal coacervate droplets in larger volumes, which resulted in the formation of more evenly coated microcapsules. Although the release of theophylline from the microcapsules was found to fit both the first order and diffusion controlled release processes, differential rate treatment indicated that the overall release, in fact, was governed by diffusion controlled process.