Abstract. The influence of the coverage of the kaolin surface with humic acid on its structural properties has been investigated. Humic acid (HA) was extracted with water from Ah horizon of an acid forest soil. The particle size of kaolin <2 μm were separated by suspension centrifugation, and kaolin consisted of 70% kaolinite. Ali samples were in powdered forms and, in addition, some selected samples were used as pellets. Textural analyses were performed by Carlo Erba Mercury Porosimeter Series 2000. The cumulative pore size distribution and the pore size distribution (PSD) curves for kao­lin and kaolin with HA were analysed. Bulk density, surface area, average pore radius and the total porosity were calculated using cylindrical pore model. The study confirmed that humic acids play an imprtant role in the structure formation of kaolin. In powdered samples, the cumulative pore size distributions curves split into three groups having different pore volumes. This proved the existence of a dependence of pore volume on the added amount of humic acid. The lack of proportionality be-tween the total pore volume and the humic acid concentration observed in this study may also indicate inhibiting influence of intermediate humic acid doses on flocculation. In kaolin which was initially wetted with distilled water, a fast decrease of total pore volume was observed. This may be explained by existence of mineral flocculation in distilled water.