Poly(acrylic acid) (PAAc) and poly(vinyl chloride) (PVC) were treated with an argon plasma to create unsaturated bonds at the surface. By use of X-ray photoelectron spectroscopy and Fourier transform infrared measurements, it was shown that the pendant groups of these polymers are removed by the argon plasma treatment. This resulted in the formation of unsaturated bonds and cross-links in the modified layer. It was found that the removal of the pendant groups is induced by UV light emitted by the argon plasma. During treatment of PAAc decarboxylation took place, which made the argon plasma more oxidative in character. The modified layer was reoxidized and eventually the PAAc surface was ablated with time. The removal of chlorine from PVC was found to be preferential, and a highly cross-linked layer, containing at least 15% unsaturated bonds, was obtained. The outermost top layer of this modified layer became oxidized after exposure to air due to a reaction between long-living radicals and oxygen.