An analysis was done of changes in the crystallinity, the oxidation induction time (OIT) and the degree of surface oxidation of polyethylene thin films before and after gas-phase oxidative surface modification. Various surface treatments were performed which allowed to differentiate between oxidation of the bulk and the surface. It was shown that for a full characterization of oxidation processes in solid films all three parameters (degree of crystallinity, surface energy and OIT) needed to be taken into account. The correlations between the degree of crystallinity and OIT can be used as an indicator of the extent of bulk modification. The correlations between the surface energy and the OIT can be used as an indicator of surface oxidation.