Abstract <p>Humic acids form various coagulation structures when the content or chemical composition of the dispersion medium changes. The micelle-like organization of humic acids is characterized by the critical micelle concentration. Humic acid molecules spontaneously aggregate through the folding of their polymer chains. The colloidal properties of humic acids isolated from peats of different genesis were determined. Surface tension isotherms of humic acids obtained by stalagmometry and the Du Noüy method have the characteristic form typical of surfactants. The following parameters were calculated: critical micelle concentration, surface activity, and Gibbs energy of micellization. A new parameter, the degree of association, was proposed for characterizing the colloid-chemical properties of humic acids. The dependences of the extinction coefficient of humic acid solutions on their concentration, pH, and ionic strength were studied; these dependences make it possible to assess the effect of these factors on the degree of molecular association. A method for determining the reducing capacity of humic acids with the Folin–Denis reagent instead of the extinction coefficient is considered, which increased the sensitivity of the method and reduced errors. The reducing capacity of humic acids increases at the same concentrations as the extinction coefficient, confirming the applicability of the proposed method. The extinction coefficient of humic acid solutions varies directly with pH and ionic strength because humic acid aggregates disintegrate due to electrostatic repulsion between dissociated protogenic groups. Solution acidity has the strongest effect on the coagulation of humic acids compared with ionic strength, indicating the absence of interaction between the indifferent electrolyte and the functional groups of humic acids.</p>