Argon plasma treatment followed by exposure to atmospheric oxygen has been used to introduce new carbon–oxygen functionalities to polymer surfaces. We report on the treatment of poly(styrene) (PS), low density poly(ethylene) (LDPE), poly(propylene) (PP) and poly(ethylene terephthalate) (PET). Argon plasma-treated polymer surfaces exhibit a ‘saturation’ level and a ‘stable’ level of oxygen incorporation. The former is the maximum amount of oxygen which can be introduced into the surface. The latter is the maximum level of oxygen incorporation at which the surface is stable to both washing with a polymer non-solvent, and ageing with time. PS exhibits the greatest level of stable oxygen incorporation followed by LDPE and PP. PET displays very little stable oxygen incorporation. ‘Stable’ surfaces are characterised by a high selectivity towards C–O functionalities (>70% for PS at O/C ratio=0.2). At least 40% of these functionalities have been shown to be hydroxy groups through chemical derivatisation. The O 1s/O 2s ratio has been used to highlight differences in modification depths between the hydrocarbon polymers. We think that the inert gas plasma treatment imparts reactivity to a surface through an energy transfer process. Our results are rationalised on the basis of the accepted mechanisms of cross-linking vs. chain scission for the polymers.