The present study was conducted on 6 different peaty-moorsh soils. The studied moorshes belonged to two groups: peaty moorsh – Z₁ and proper moorsh – Z₃. They represented soils of weakly, medium and strongly, very strongly secondary transformed states. The quantitative degree of the secondary transformation of the investigated materials was expressed by W₁ index determined according to Gawlik method. The adsorption and desorption isotherms were determined based on experimental data of the water vapour adsorption at constant temperature (20°C). For evaluation of the specific surface area and adsorption energy the model based on assumption of polymolecular adsorption (BET theory) had been applied. All investigations were performed on natural and monoionic (H⁺, Ca²⁺) forms of studied peaty-moorsh soils. The shape of all adsorption isotherms was similar and reflects the second type in BET classification. The results showed a positive correlation between the specific surface area and the state of secondary transformation. Furthermore, the specific surface area values obtained for natural forms were higher than for monoionic forms.