A fluorescence method was adapted to measure moisture-uptake kinetics in films of poly(methyl methacrylate), and data were interpreted in the context of a Fickian diffusion model. The films, 2-60 mum thick, were supported on acid-etched microscope slides. They were compared with several freestanding slabs about I mm thick. The moisture diffusion in the slabs was Fickian with a diffusivity of 3.2 X 10(-9) cm(2)/s. The apparent Fickian diffusivity in the films decreased substantially with decreasing film thickness; however, a careful examination revealed that the initial moisture uptake was governed by a thickness-independent diffusivity for a wide range of film thicknesses. This suggested that the appearance of non-Fickian behavior originated within about a micrometer of the buried interface, possibly as a result of water accumulation beneath the film or slight thickness variations. Moisture uptake in the thickest films was more rapid than in the slabs, most likely because of residual thermal stresses.