Low density polyethylene (PE) and polyethylene terephthalate (PET) films were treated in air plasma of a low pressure (500 mTorr) large area microwave (2.45 GHz) discharge, or in a corona discharge at atmospheric pressure. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) for their oxygen content [O] and surface chemical structure, which were related to the corresponding peel strength of PE/PE and PE/PET laminates. Although the oxygen concentration at the surface increased monotonically with the degree of treatment, the peel force reached a maximum and then sharply decreased. Regardless of the treatment type, the maximum peel force occurred for [O] values between 10-15 at.%. The highest peel force has been found to occur when the concentration of C-O (hydroxyl, ether, epoxy,...) groups is highest and that of O=C-O (carboxyl) groups is lowest, which corresponds to the situation when the effect of a weak boundary layer, due to low molecular weight materials, is minimal (low O=C-O concentration).