The dissociation behavior of humic and fulvic acids was characterized on the basis of conductometric measurements. It was confirmed that dissociation is strongly affected by the content of humic or fulvic acids in aqueous solution. In the case of fulvic acids, the dilution effect connected with the increase of dissociation degree and the values of activity coefficients was the main factor influencing their dissociation ability. In the case of humic acids, their aqueous leachates were used and characterized as the main fractions functioning actively in natural systems. These fractions can be rich in acidic functional groups and the chemical compositions of these fractions change with their contents in the leachates. The amount of the dissolved humic fraction, its chemical character and its dissociation ability can be considered to result from equilibrium between dissolved and undissolved humic substances. A maximum was observed for the dependence of the dissociation degree on the concentration of humic leachates. In less concentrated leachates, less soluble humic particles with weaker functional groups were able to be dissolved. 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 an increase in the degree of dissociation up to a maximum. In the case of fulvic acids, the dissociation degree increases with decreasing concentration as a result of the dilution. Two different values of pKa were determined for each humic and fulvic sample. The first was determined for higher concentrations, where pKa values are practically constant. The second was the value extrapolated to infinite dilution.