Surfaces of poly(isobutylene) (PIB) and poly(butylacrylate) (PBA) pressure-sensitive adhesive tapes were treated by oxygen plasma, and effects of surface modification on their adhesive behavior were investigated from the viewpoint of peel adhesion. The peel adhesion between PIB and PEA pressure-sensitive adhesive tapes and stainless steel has been improved by the oxygen plasma treatment. The surface-modification layer was formed on PIE and PEA pressure-sensitive adhesive surfaces by the oxygen plasma treatment. The oxygen plasma treatment led to the formation of functional groups such as various carbonyl groups. The treated layer was restricted to the topmost layer (50-300 nm) from the surface. The GPC curves of the oxygen plasma-treated PEA adhesive were less changed. Although a degradation product of 1-3% was formed in the process of the oxygen plasma treatment of the PIE adhesive. There are differences in the oxygen plasma treatment between the PIB and PBA adhesives. A close relationship was recognized between the amount of carbonyl groups and peel adhesion. Therefore, the carbonyl groups formed on the PIE and PEA adhesive surfaces may be a main factor to improve the peel adhesion between the PIE and PEA adhesive and stainless steel. The peel adhesion could be controlled by changing the carbonyl concentration on the PIB and PEA adhesive surfaces. We speculate that the carbonyl groups on the PIB and PEA adhesive surface might provide an interaction with a stainless steel surface.