A bleached softwood kraft pulp was treated with fluorescent whitening agents Tinopal UP and Leucophor AP under different conditions. The kinetics of adsorption-chemical interaction between optical brighteners and pulp was described by an exponential kinetic equation. An entropy heterogeneity of the pulp surface related to the active sorption centers, their accessibility and their spatial orientation was confirmed. The activation energy values obtained were found independent of the brightener concentration and on the increase of pulp whiteness. A decisive role of the entropy factors was ascertained.