A b s t r a c t. The aim of this work was to study how different salt and surfactant solutions influence the particle size distribution and colloidal stability of sandy soil extracts. Particle size distribution was investigated by the laser diffraction method. Extracts were made from the soil – before and after removing its organic content – with solutions of NaCl or CaCl₂ and one cationic and two anionic surfactants. The surfactants influence the particle size distribution of the soil. Due to the use of the NaCl and surfactant mixtures after removal of organic content, the particle sizes increased compared to the extract of the soil. Colloidal stability was investigated by the laser Doppler electrophoresis method resulting in a zeta potential between -5.63 and -23.7 mV, showing that the extracts were rather instable. Static equilibrium experiments with sodium dodecyl sulphate on sandy soil resulted in an L-type of isotherm with three steps, indicating the formation of more surface layers. Comparison of the adsorption isotherm and the measurements of particle size distribution demonstrated that the particle size changes comparably with the formation of the different layers. The zeta potential of the equilibrated solution reached the region of instability and stability when the initial concentration of sodium dodecyl sulphate was near its critical micelle concentration.