A research team has used chlorine dioxide to oxidize polypropylene. Under LED irradiation, ClO2* radicals attacked the methyl groups of polypropylene, converting them to carboxylic acid. The C-H bond-breaking was selective to the side chain. The oxidized surface could be stained with cationic dyes. Surface oxidation of plastics is industrially important, but existing methods are polluting or poorly controlled. This clean, convenient process could produce functionalized plastics for medical or printing uses.