A new method for surface treating polymers and polymer composites based on gas phase sulfonation has been shown to quickly and effectively increase wettability and adhesion to epoxy. A gas mixture containing a low concentration of sulfur trioxide in nitrogen (similar to 1% v/v) was used to treat the surfaces of polypropylene and polystyrene films. The sulfonated surfaces were then neutralized with ammonium hydroxide. The effectiveness of sulfonation on the adhesion of these polymers to an epoxy adhesive was investigated using mechanical testing of sandwich lap-shear specimens. The lap-shear adhesive joint strength of epoxy to sulfonated polypropylene was compared with polypropylene treated with currently accepted surface treatments including chromic acid etching and flame treatment. Sulfonation greatly improves the adhesion of polypropylene to epoxy compared with other surface treatment techniques as measured by lap-shear strength. An optimum sulfonation treatment level was shown to exist for polypropylene. For polystyrene surfaces, it was shown that while sulfonation immediately increased wettability, it did not greatly improve its adhesion to epoxy; it did, however, significantly reduce the scatter in the ultimate strength values. Excess sulfonation treatment reduced the lap-shear strength for both polymers. X-ray photoelectron spectroscopic examination of the locus of failure of tested joints has shown that failure occurs in a weak boundary layer for these surface-treated polymers.