The thermodynamics and mass transfer aspects associated with the use of CO2 for the acidification, particularly for the removal of Na+ from pulp fibres, were studied. The acidifications with CO2 and with H2SO4 were carried out in a stirred tank reactor under different temperatures, agitation speeds, concentrations of CO2 and H2SO4 and treatment times. The CO2 acidification of pulp was found to be very fast at Re > 10(4) and N-p of 4, and reached equilibrium within 5 min. The volumetric mass transfer coefficient, k(L)a, of the CO2 acidification was found to be in the range of 0.0005-0.0083 s(-1). Thermodynamically, the CO2 acidification was achievable only at pH > 4. The extent of the resultant Na+ exchange increased with the acidification time, agitator speed and CO2 flow rate. Under similar experimental conditions, the extent of Na+ desorption by CO2 acidification was found to be slightly lower than that achieved by sulfuric acidification. The equilibrium distribution of Na+ in the solution and the fibre phase can be estimated using a model based on the Donnan theory. (C) 2009 Elsevier B.V. All rights reserved.