Kraft lignins are known to associate and form large clusters in aqueous solutions at pH values close to pKa. Some physicochemical properties related to association, such as the amount of free phenolic groups and hydrodynamic radius, have been determined in ultrafiltrated fractions of leached softwood kraft lignins and Indulin AT. Average diffusion coefficients were measured by the 1H PFG-NMR method in 2 % w/w lignin solutionsof 0.1 M LiOH/D2O. The hydrodynamic radius were calculated using the Stoke-Einstein expression. Conductometric titrations were used to determine the average amount of free phenolic groups in the fractions. The repulsive electrostatic potential, F0, originating from the phenolic charges, was calculated at different pH values for one of the fractions, using the Poisson-Boltzmann cell model.