Carboxymethyl guar gum (CMGG) was synthesized by carboxymethylation of guar gum (GG), which was blended with gelatin (GE) to obtain a novel semi-interpenetrating polymer network (semi-IPN) in the form of microspheres prepared by water-in-oil emulsion method to investigate the controlled release of theophylline (THP), an antiasthmatic drug. Electronic spectroscopy revealed the drug encapsulation ranging from 56 to 74 %. Fourier Transform infrared spectroscopy confirmed the carboxymethylation of GG as well as the semi-IPN structure of the blend polymer. Scanning electron microscopy indicated the smooth surfaces with spherical microspheres. Differential scanning calorimetric and X-ray diffraction studies showed the molecular level dispersion of drug in the microspheres. The in vitro drug release profiles were analyzed to study the effect of polymer blend composition, % drug loading and amount of glutaraldehyde added as a crosslinker. The drug release was extended up to 26 h. The in vitro release data performed in acidic and alkaline media were analyzed using the empirical equations to understand the release profiles of THP.