pH sensitive copolymeric hydrogels have been synthesized by free-radical polymerization of methacrylamide and acrylic acid in aqueous medium. The gels were characterized by FTIR spectroscopy, thermogravimetric analysis, and shelling measurements. To determine the suitability of theses hydrogels for gastrointestinal oral delivery of model drug theophylline, their swelling behavior was investigated as a function of pH and various structural parameters such as the average molecular weight between crosslinks, crosslink density, and mesh size were calculated. Likewise initial, average and late time diffusion coefficients were also evaluated in simulating intestinal fluid of pH 6.8 at 37 C. The gel underwent sharp volume phase transition in the vicinity of pH 5.8. The mesh sizes of the hydrogel were between 8.4 and 9.2 angstrom in the collapsed state (pH range 1-2; SGF) and between 514 and 524 A in the swollen state (pH range 7-8; SIF). The experimental data was found to fit well to Beren-Hopfenberg equation thus suggesting that later part of swelling was chain relaxation controlled. The activation energy, as determined from Arrhenius equation was found to be 13.71 kJ mol(-1). Likewise, enthalpy of mixing was also evaluated using Gibbs-Helmholtz equation. (c) 2006 Wiley Periodicals, Inc.