the present invention provides a method of modifying the surface of a polymeric substrate , e . g ., to improve the wettability of the polymer film surface and / or alter the reactivity of the surface of the substrate by further oxidation , comprising exposing the substrate to a flame . the flame is supported by a fuel and oxidizer mixture that includes an effective amount , for modifying the polymeric substrate , of at least one oxygen - containing compound that functions as a fuel substitute . oxygen comprises between about 10 and 50 atomic percent of the compound . large increases in the astm wetting test over that reported with conventional flame - treating processes , have been observed in films treated according to this invention . in addition , significant increases in polymer film surface oxidation levels have been observed .