Semi-interpenetrating polymer network (semi-IPN) hydrogel blend microspheres of gelatin and hydroxyethyl cellulose were prepared by a water-in-oil (w/o) emulsion technique and used to investigate the controlled release (CR) of theophylline (THP), an antiasthamatic drug. About 74% encapsulation of THP was achieved, and the drug release profiles were analyzed in terms of gelatin and hydroxyethyl cellulose blend composition, amount of cross-linking agent, and percentage drug loading. Fourier transform infrared (FTIR) spectroscopy confirmed the formation of the IPN blend matrix, as well as chemical stability of the drug in the microsphere. The physical state of the drug in the IPN matrix as evaluated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) remained undisturbed. The size of the microspheres varied from 98 to 144 mu m as measured by laser light scattering. Scanning electron microscopy (SEM) indicated the smooth surface morphology of the microspheres. Equilibrium and dynamic swelling of the microspheres in distilled water were measured to compute the diffusion coefficient (D-v) of the drug solution through the microspheres. The in vitro cumulative release data were analyzed using an empirical equation to compute the diffusion exponent (n), whose values suggest a non-Fickian mode of transport.