The intrinsic microporosity of hydrolyzed polymeric films was studied by means of gas sorption using nitrogen and carbon dioxide as probe molecules. X-ray scattering was used to get a better insight into the polymer microstructure. The results show that the polymer chain stiffness is not affected by the degree of hydrolysis, it can be assumed that the reduction of the size of the free-volume elements is due to chain deformations. The presence of this hysteresis points to a much denser structure compared to the initial samples. The unmodified PIM-1 exhibit microporosity as determined by nitrogen sorption at 77 K, and PIM1s modified with carboxylic acid groups did not take up nitrogen, except they were precipitated from solution. Temperature-dependent SAXS measurements show that microstructure of the carboxylated polymer films is changed upon heating.