The diffusion of pure liquid water into a commercial polypropylene (PP) film at 278-348 K was studied with Fourier transform infrared attenuated total reflectance spectroscopy. Abnormal diffusion behavior was indicated by a significant deviation between the experimental data and a Fickian diffusion model with the conventional saturated boundary condition applied at the water/PP interface. This deviation was observed at all the temperatures studied. With a modified boundary condition that took into account a mass-transfer resistance at the water/PP interface, the Fickian model was able to represent the experimental data satisfactorily. The average water diffusion coefficient varied between 1.41 and 7.64 x 10(-9) cm(2)/s, with an activation energy of diffusion of about 19.3 kJ/mol. The interfacial mass-transfer resistance was represented by an exponential model with an empirical relaxation parameter. The relaxation parameter beta increased as the temperature increased and reached an apparent plateau. The infrared spectrum indicated a positive chemical shift of 18 cm(-1) for the less strongly hydrogen-bonded component of the broad hydroxyl stretching band with respect to pure liquid water, indicating that hydrogen-bonding interactions were weakened or broken when water molecules diffused into the PP matrix.