Poly(methacrylic acid) (PMAA)-based pH-sensitive hydrogels are widely used for biomedical and pharmaceutical fields because of their unique swelling properties and highly hydrated structure. To better serve various applications, it is necessary to have a thorough understanding of the reaction mechanism and the structure formation of synthesized hydrogels. In this study, the PMAA hydrogels crosslinked with tri(ethylene glycol) dimethacrylate (TEGDMA) were synthesized using free radical photopolymerization in a water/ethanol mixture. The effect of light intensity on the reaction kinetics and structure formation was addressed through a series of analytical methods, including photo-differential scanning calorimetry (PhotoDSC), photo-rheometry, and dynamic light scattering (DLS) goniometry. The copolymerization of the photocurable MAA/TEGDMA system was enhanced as the light intensity increased at low resin conversion. However, the light intensity decreased the reaction rate at high resin conversion. The use of a high light intensity significantly shortened the reaction time to reach macrogelation and increased the swelling ratio of formed hydrogels, which can be explained by the relative rates of intra- and intermolecular reactions during the microstructure formation. (C) 2007 Elsevier Ltd. All rights reserved.