The surface of polypropylene (PP) film was oxidized by exposure to a flame fueled by isotopically labeled methane (CD4). The isotopic sensitivity of static secondary ion mass spectrometry (SIMS) was then used to gain new insights into the mechanism of flame treatment. SIMS analysis indicated that much of the oxidation of PP occurring in fuel-lean flames is not deuterated, while for PP treated in fuel-rich flames, some of the affixed oxygen is deuterated. These observations imply that O-2 is the primary source of affixed surface oxygen in fuel-lean flame treatments, but that OH may be a significant source of affixed oxygen in fuel-rich flame treatments. Hydroxyl radicals are primarily responsible for hydrogen abstraction in fuel-lean flames, while H is the primary active gas-phase species in fuel-rich flames. SIMS also detected trace quantities of oxidized nitrogen groups affixed to the flame-treated PP.