In this study the author made an attempt at the determination of the character of the surface of humus acid and clay mineral complexes, with iron and calcium acting as bonding agents, taking into acount the soil pH, by means of methods based on water vapour adsorption. For the purpose, under laboratory conditions, humus acid preparations (humic acid and b-humus fulvic acid fraction) were made, at various quantitative ratios, with clay minerals (micamontmorillonite, Na-montmorillonite, kaolinite), in the presence of Fe and Ca, at pH 4 and 2.5. (i.e., at pH values close to humus acids precipitation from the solution). The nature of the surface of the preparations was characterized by means of analysis of the shapes of water vapour adsorption isotherms at 20 and 40°C, and by analyzing such physical parameters as the specific surface area and the molar energy of adsorption, those properties perfectly characterizing the surface of an organic-mineral adsorbent.