Model polypropylene and polypropylene/low molecular weight modifier systems with identical crystalline structure but of different physical state of noncrystalline regions were analyzed. The deformation of reference material was accompanied by a cavitation phenomenon while the deformation of the polypropylene/modifier systems occurred in non-cavitating manner. Based on X-ray and PALS measurements the observed change of the intensity of the cavitation phenomenon during the deformation of the analyzed systems was explained. Additionally, the change of interlamellar distance (induced by introducing the modifier molecules and uniaxial stretching) was correlated with the change of average size of the free volume pores of the amorphous phasethis analysis was performed based on experimental data and theoretical estimations. It was proven that the presence of modifier reduce significantly the average size of free volume pores in relation to the system with similar interlamellar distance. Finally, the method enabling specifying the effective content of the modifier in interlamellar regions based on PALS measurements and the observed change of the value of long period was presented. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 531-543