The surface properties of unbleached softwood (Pinus sylvestris) and hardwood (Betula verrucosa) kraft pulps were modified by treating the fibre surfaces with specific xylanase and mannanase (Trichoderma reesei). The amounts and accessibilities of surface carboxyl groups in untreated and enzyme-treated fibres were analyzed using adsorption of cationic polyelectrolytes and high precision potentiometric titration. According to the potentiometric titration both softwood and hardwood pulps contained two types of acidic groups, one with pK(1) approximate to 3.3 (uronic acid in xylan) and one with pK(2) approximate to 5.5 (probably carboxylic group in lignin), The relative amount of the weaker acid in softwood and hardwood pulp was 10% and 20% of the total acid content, respectively. The ratio of the two acids in pulps was not changed bq mannanase treatment, while xylanase treatment decreased the relative amount of the stronger acid. This indicates that the major part of the carboxylic groups are bound to xylan, Xylanase treatment removed about 30% of the xylan in both birch and pine pulps and, consequently 22% of the acid groups in hardwood pulp but only 8% of the acid groups in softwood pulp. Hence, in softwood kraft pulp fibres the content of uronic acid side-groups in accessible xylan on the surfaces of the fibres is lower than that on the hardwood fibres.