In a binary system the sorption of Na+ and Ca2+ by kraft pulps follows a second order kinetics, and the isotherms follow the Langmuir model. The sorption capacity is dependent on pH and temperature. The thermodynamic parameters indicate that the sorption is exothermic and spontaneous. In a single-component system, the sorption capacity of the kraft pulp for Na+ and Ca2+ increases with the degree of dissociation of ionisable cellulosic components of the kraft pulp fibres. In a binary system the exchange of Na+ and Ca2+ is highly dependent on pH. SEM and X-ray analysis indicated that at pH above 11.0 calcium forms a precipitate bound to the surface of the fibres. (c) 2006 Elsevier Inc. All rights reserved.