The fluorescent molecular rotor probes 4-tricyanovinyl-[N-(2-hydroxyethyl)-N-ethyl]-aniline, tricyano-4-(dimethylamino)benzylidene, and tricyanovinyljulolidene have been used as extrinsic fluorescence probes for quantitative monitoring of water uptake in polymeric coatings. The presence of water causes plasticization of the polymer, which results in increased local mobility within the film. The nonradiative decay pathways of the rotor probes are increased as local mobility increases, and the resulting decrease in fluorescence intensity of the probes is directly proportional to the amount of water sorbed. Beyond allowing for the characterization of sorbent content, this fluorescence technique can be used to determine the diffusion coefficient of water in a polymer film. The relative change in fluorescence fits well to a Fickian diffusion model, yielding a diffusion coefficient for water of 3 X 10(-8) cm(2)/s in poly(vinyl acetate), and a value of 6 X 10(-9) cm(2)/s in a room-temperature cured epoxy-polyamide, in excellent agreement with diffusion coefficient values determined from gravimetric analysis. Preliminary studies also demonstrate the utility of molecular rotor probes to monitor water uptake in individual layers of multilayered polymer systems.