Optical brighteners Tinopal UP and Leucophor AP are used in additional treatment of bleached softwood kraft pulp under different conditions. The present paper describes the applicability of Temkin adsorption isotherm to characterize the thermodynamic equilibrium reached in chemisorption of an optical brightener on pulp's uniformly non-homogeneous surface. It is found that this is an endothermic process. The entropy factors dominate the developing non-homogeneity and hence the variation of the Gibbs's free energy as well. (c) 2007 Elsevier Ltd. All rights reserved.