Isotherms of water vapour adsorption were analyzed and specific surface area of humic acids and functions of β-humus fulvic acids were determined. Humic acids were separated from soil 4 years after using sewage sludge and vermicompost in doses of 30, 150, 600 Mg ha^–1. The Control was the nonfertilized soil, as well as the soil fertilized with manure in the dose of 30 Mg ha^–1. Humic acids were separated using Schnitzers method, while after their separation, fractions of β-humus fulvic acids were precipitated in the supernatant using Stevensons method. The obtained results indicate that the used waste substances bad a modifying effect on the magnitude of vapour adsorption by humic acids. In the case of humic acids it was an increase of adsorption, while for fractions of β-humus fulvic acids, the adsorption decreased. The specific surface area of β-humus fractions is much greater than the one of humic acids. The decrease of the specific surface area of the analyzed humic acids was influenced most significantly by the dose o f 600 Mg ha^–1 of sewage sludge.