This study investigates the chemical changes effected on the surface of polypropylene and polystyrene as a result of sulfonation using an SO3-air mixture (1% SO3 v/v). Polypropylene is a purely aliphatic system and is characterized by the tertiary carbon which it has on its backbone. On the other hand, polystyrene is characterized by an aromatic ring on its backbone. Different surface analytical techniques (XPS, FTIR-ATR, and contact angle measurements) were used to quantify the changes. It was concluded that the formation of sulfonic acids was the main chemical change which occurred on the surfaces of polypropylene and polystyrene. Besides sulfonic acids, there was an indication of several side reactions occurring in the case of polypropylene while polystyrene did not show any such side reactions. The surfaces of both polymers reached a state of treatment saturation (similar to 6.5% for polypropylene and 7% for polystyrene). Sulfonation of polypropylene beyond the saturation limit led to degradation, as manifested by a highly sulfonated layer of polypropylene which sloughed off during neutralization. However, intense sulfonation of polystyrene did not show any conspicuous degradation.