Self-aggregation of kraft lignin in aqueous solution at various conditions was studied by light scattering. The tendency for kraft lignin to self-aggregate was found to increase either when the acidity, the ionic strength or the temperature was increased. From cryo-TEM analysis it was shown that the particle clusters formed exhibits fractal structures. By studying the kinetics of aggregation it was possible to distinguish between two different modes of aggregation from the fractal dimensions of the kraft lignin particles formed. The reaction limited cluster-colloid aggregation (RLCA) process creates denser particles whereas the diffusion limited cluster-colloid aggregation (DLCA) gives particles of looser shapes. Regarding the sensibility towards self-aggregation and precipitation by various monovalent electrolytes, the Hofmeister series for cations was found to be valid for kraft lignin. For anions, however, kraft lignin showed a reversed order of sensibility towards self-aggregation and precipitation compared with the Hofmeister series. The stability against self-aggregation was seen to increase dramatically when small amounts of bile acid salts were added to kraft lignin solutions. The outcome of these studies shows that in order to understand the solution behaviour of kraft lignin, sometimes kraft lignin should be treated as a macromolecule and sometimes as a colloid.