The effect of phase composition and molecular mobility on oxygen permeability is studied for stretched films prepared from polyamide 6 (PA6) and a blend of PA6 with semiaromatic amorphous polyamide (aPA)-PA6/aPa, which is a random co-polyamide containing iso- and terephthalic chain units. The effect of stretching degree of films on crystallinity, and strain-induced immobilization of the amorphous phase is studied by DSC and solid-state proton NMR transverse (T-2) magnetization relaxation, respectively. Changes of these parameters upon strain are determined. Crystallinity and T-g are hardly affected by strain, as shown by DSC. Molecular mobility in the amorphous phase of stretched films is largely restricted upon increasing strain. At temperatures well above T-g, the amorphous phase consists of two fractions of nanometer thickness: one behaves like glassy polyamides, and the other one, a semirigid fraction, reveals largely hindered chain mobility. The amount of the glassy-like fraction increases proportionally to the strain. In addition, molecular mobility in the semirigid fraction decreases with increasing strain. According to high-resolution solid-state C-13 NMR experiments, the composition of the semirigid fraction in PA6/aPA films is enhanced by aPA. It is shown that the immobilization of the amorphous phase has a large influence on the permeability of the films. It appears that oxygen permeability correlates well with a parameter describing strain-induced decrease in molecular mobility in the amorphous phase. This parameter is the reciprocal product of the amount of the semirigid fraction at temperatures well above T-g and molecular mobility in this fraction as determined by NMR T-2 relaxation time. It is suggested that the semirigid fraction of the amorphous phase could be considered as "channels" for diffusion of oxygen molecules, Despite lower oxygen solubility in PA6 films, as shown by low-temperature proton NMR T-1 relaxation data, the permeability of all PA6/aPA films under humid conditions is significantly lower than that of PA6 films. It is suggested that the lower permeability of PA6/aPA films is due to complex formation between oxygen molecules and aromatic rings of aPA, which slows down oxygen diffusion. Results of the present study are of interest for a better understanding of both permeability of polyamide films and other phenomena in polyamides that are affected by transport properties of small molecules, such as the rate of water uptake, dyeability of fibers, thermal oxidation, and blooming.