The rheological and morphological properties and in vitro theophylline release of tertiary amine-based microgels were evaluated. The testing of such a formulation through in vitro diffusion experiments revealed that the release of theophylline from the microgels was pH-dependent and differs significantly with respect to a nonresponsive gel like scleroglucan (Scl). The microgels were obtained from 2-(diethyl amino) ethylmethacrylate (DEA) in the presence of a bifunctional crosslinker at pH 8-9. As the resulting microgels are pH-responsive and an increase in viscosity from high to low pH range is exhibited, the in vitro release of theophylline as model drug was studied at different pHs of both the matrix and the receptor medium. The release behaviors of PDEA-based microgels were compared to nonresponsive natural gel Scl, studied previously. For microgels, diverse release patterns were found at different acidity conditions. This observation seems to be related to complex diffusion phenomena and the different gel structure obtained for samples prepared at dissimilar pH. (C) 2007 Wiley Periodicals, Inc.