The effects of solvents on the surface properties of oxygen plasma-treated polyethylene and polypropylene films have been studied by ESCA, contact angle measurement, and adhesion testing. The results show that oxygen plasma treatment produces some low molecular weight materials (LMWM) on the treated surfaces, which can be removed, to some extent, by solvents. It seems that the LMWM has different solubilities in different solvents. Among the solvents (water, acetone, and 2-propanol) used, acetone has the most significant effect. The removal of LMWM considerably reduces the wettability of the treated materials, but does not impair the adhesion increased by the plasma treatment.