Free-radical photocurable hybrid sol-gel materials have gained special interest during the last decades. Compared to thermally processed materials, they present the advantages of fast curing, low energy consumption, and spatiotemporal control of the reaction. Although comprehension of the photochemical step is fundamental, little is known about the characteristic of photochemistry in this kind of material. Real-time Fourier transform infrared spectroscopy was used to study the photopolymerization of a hybrid sol-gel upon ultraviolet irradiation. Various photoinitiator systems were tested for their efficiency in inducing the polymerization of pendant polymerizable moieties anchored on a partially condensed silicate network. The presence of O-2 and the nature of the polymerizable function were shown to be crucial factors in the photoinduced process. The effects of the photoinitiator concentration and light intensity were also studied. These results were explained in terms of classical kinetic models developed for all-organic photopolymers to point out the distinctive aspects related to the use of photoinitiated polymerization in hybrid sol-gel materials. (C) 2003 Wiley Periodicals, lnc. J Polym Sci Part A: Polym Chem 41: 831-840, 2003.