We describe a general method of finding the transport properties of molecules in polymer films by photolysis. Poly(methyl methacrylate) samples held at different temperatures are exposed to UV radiation at discrete wavelengths, and the time evolution of the volatile photoproducts are detected with a quadrupole mass spectrometer. A diffusion model is used to fit the experimental data and deduce diffusion coefficients for the main photoproduct methyl formate. The average value at room temperature was equal to 1.9 x 10(-11) cm(2) s(-1) at all wavelengths investigated. Together with the values derived at other temperatures, an Arrhenius plot was obtained and the activation energy for methyl formate diffusion within the polymeric thin film calculated from the slope of the graph. We envision that this new method will find application to a variety of problems involving the mass transport of molecules through boundary layers of single or multilayer thin film structures.