The large sources of natural organics, which are important components of not only soil but also aquatic environments, are mucks. Soil organic matter takes part in all processes coupled with acid –base properties. OM is involved in a number of biogeochemical processes, including pH buffering, nutrient cycling, ionic balance, mineral weathering, metal leaching, pollutant toxicity, mobility and bioavailability. Functional groups of acidic character dominate on soil organic matter surfaces. These groups (aliphatic and aromatic carboxylic, phenolic etc.) have very different acidic strength, depending on the kind of the group, its locality. The number and acidic strength of these groups are a primary characteristics of soil organic matter. The study was conducted on five muck samples (Terric Histosols) being at different state of mucking, collected at depth of 5-20 cm from sites located in a low moor area of the Wieprz-Krzna Canal (Polesie Lubelskie) and Biebrza river in Poland. The suspensions of the samples in concentrated NaCl solution were back titrated with 0.1M NaOH from pH=3 upward. The titration curve of pure NaCl solution was taken as the titration curve of the equilibrium solution. From the difference of above curves the base consumption by the solid phase vs. pH was estimated. The latter was assumed to be equivalent to variable surface charge increase of the solid. From these dependencies the distribution functions and average values of apparent surface dissociation constants were calculated. The results obtained allow to state that studied mucks have different quantitative and qualitative characteristics of surface functional groups. In all investigated mucks two dominating kinds of the acidic groups were distinguished; i.e. strongly acidic surface sites of low pK values (below 4) and weekly acidic surface sites of high pK values (above 8). The amount of medium acidity surface groups is small in all samples. The relationship between the variable charge and the state of mucking process was found. The more decomposed the sample the more acidic the surface and the higher the variable surface charge per unit mass.