High-density polyethylene (HDPE) and low-density polyethylene (LDPE) films were oxidized by treatment with ozone and by photooxidation with a low-pressure mercury lamp. The changes that resulted in the surfaces of the films were followed by ESCA. On ozonation, the surface of LDPE initially is oxidized more rapidly than that of HDPE; however, extended ozonation produces a surface composition that corresponds to C8O for HDPE and to C18O for LDPE. The surface oxidation products are mainly carboxyl groups, with lower levels of carbonyl and C[BOND]O groups. For both polymers photooxidation provides more extensively oxidized surfaces than ozonation, although the surface of HDPE oxidizes slightly faster than that of LDPE treated under identical conditions. In both cases the surface stoichiometry after extensive photoxidation is C6O. The functional groups formed are mainly carboxyl and C[BOND]O. The effects of ozonation and photooxidation on the polyethylene surfaces are compared with those produced by several other means of surface oxidation.