A cell is described for characterizing the diffusion of small molecules through thin polymer films using attenuated total reflectance (ATR) Fourier-transform infrared spectroscopy. The cell is designed to be used with precast (commercially extruded) polymer films, removing the need to cast the film directly onto the ATR crystal and allowing the as-processed transport properties of the film to be assessed. Pressurized gas is used to maintain contact between the polymer film and the ATR crystal during the diffusion experiment. Data are presented to demonstrate the use of this cell for measuring the diffusion of a gas (CO2), two liquids (amyl acetate and limonene), and simultaneous diffusion of individual components from a liquid mixture (50/50 amyl acetate/limonene) through thin polymer films. In the last case, it was found that pure penetrant transport rates do not necessarily apply to diffusion of the same penetrants from a mixture. Diffusion coefficients obtained from the ATR cell compare favourably with values obtained gravimetrically for the same polymers and penetrants.