The use of plastic films with specific diffusion or permeation properties for industrial applications has grown at a considerable rate. Some useful applications are found in medical devices, bioreactors, and combustible fuel storage where polymer films function as separation membranes that allow permeation of different gases at different rates. In this work, the permeation and diffusion properties of a polyester-based thermoplastic polyurethane (TPU) were investigated. TPU injected and extruded specimens were subjected to thermal treatment (annealing) at 100 degrees C for 20 h. Injected samples were exposed to certain hygrothermal conditions and films were prepared to evaluate the influence of annealing on the permeation of gases. In order to achieve a complete analysis, tests such as differential scanning calorimetry, tensile tests, and Fourier transform infrared spectroscopy were conducted to examine the morphological changes. These were then correlated to the TPU permeation behavior after annealing. Water uptake by the polymer-measured as weight gain-likely indicates an increase in the free volume in the amorphous domains. Similarly, in permeation and water immersion tests, the diffusion rate of gases and H2O through the TPU was higher for the annealed samples when compared to those without treatment, indicating that diffusion within the polymer is dependent on the postprocessing thermal treatment. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineers