Three kraft pulps and one bisulphite pulp have been studied. The surface chemical composition was determined by means of ESCA, and water sorption was studied using a dynamic contact angle tester. The surface coverage of lignin as well as dichloromethane (DCM) extractable material was found to be considerably larger than expected from bulk measurements. The results show that DCM extractable material decreases water sorption to a larger extent than lignin. Oxygen plasma-treatment improves water sorption of pulps containing large amounts of DCM extractable material and lignin due to oxidation of these compounds. Only a few seconds of exposure is needed. After ageing the improved water sorption is unchanged for kraft pulps. However, bisulphite pulp which contains larger amount of extractable material shows a decrease in rate of water sorption.