Microspheres of theophylline, with both ethylcellulose of high and low molecular weight and also their mixtures as a coating material, were prepared using the solvent evaporation technique. No permeability through intact isolated polymer films was found. Therefore, this study investigated the drug release dependence of structure and mechanical properties of the polymer matrix. In vitro dissolution studies exhibited the square-root of time (Higuchi model) release characteristics. The size distribution of microspheres was dependent on the ratio of ethylcellulose mixtures with high and low molecular weights.