Using Cl-35 elastic recoil detection (ERD) analysis, oxygen enrichment was found on the surfaces of polystyrene and a random copolymer of styrene and acrylonitrile containing 38.7 wt% acrylonitrile. The samples were prepared in a hot press at 200-degrees-C between two silicon wafers in an air environment. The oxygen-rich layer on top of the samples was less than 20 nm thick. Heavy ion ERD seems to be a promising method to study the chemical composition of polymer surfaces and concentration profiles up to a depth of several hundred nanometres.