The function of humic acids in nature is strongly affected by their solubility and their ability to dissociate. This work is focused on the fraction of humic acids which is dissolved in water or rather the aqueous phase in humic-containing systems. This fraction can be rich in acidic functional groups which are the most active ones. The amount of the dissolved humic fraction can be considered to be the result of equilibrium between dissolved and undissolved humic substances. In this study, the concentration dependence of the dissociation constants was determined and analyzed. A maximum was observed for the dependence of the degree of dissociation on the concentration. In less concentrated leachates, less soluble humic particles with weaker functional groups were able to be dissolved. The weak functional groups have lower dissociation ability and they were able to remain in non-dissociated forms. The increase of humic contents in leachates caused smaller amounts of soluble fractions with the weakest functional groups to remain in an undissolved form, which led to the increase in the degree of dissociation. The increase in concentration of humic acids in the leachates caused the degree of dissociation decrease as a result of interactions between dissolved particles. (C) 2016 Elsevier B.V. All rights reserved.