Reactive (2, 4) and corresponding nonreactive (3, 5) fluorescent probes, based on 4-(dimethylamino)4'-nitrostilbene (1), have been employed for monitoring the photoinitiated polymerization of a series of dimethacrylates. Reactive fluorescent probes are significantly more sensitive, in particular at the early stages of the polymerization process. The observed differences in emission between reactive and nonreactive probes can be explained by assuming that covalent attachment affects the distribution of probe molecules between rigid environments, presumably highly cross-linked gel particles, and mobile monomer-rich environments. Enrichment of nonreactive probes in mobile environments is expected due to diffusion out of the cross-linking regions.