Attachment of SO3H groups onto the surface of polymers by treatment with fuming sulfuric acid is a well-known procedure. In the present study we compare the number of SO3H groups per unit area measured by a number of methods including weight gain, thickness, FT-IR transmittance spectra, FT-IR surface spectra with ATR method, sodium exchange, and pH measurement of water in contact with the polymer surface. It can be shown that, under the chosen conditions, i.e., room temperature and 32% SO3 in H2SO4, oxidation of PE is the main reaction, while sulfonation accounts for less than 20% of the weight gained during reaction. Differences in the reactivity of high density and low density polyethylenes are discussed. Deprotonation in water compared with H+ /Na+ exchange indicates a poor penetration of sodium ions into deeper layers of the polymer. © 1994 John Wiley & Sons, Inc.