Water vapour adsorption on peat-moorsh soils in various phases of the moorshing process was investigated. The soil material represented two kinds of moorsh: peaty moorsh (Z₁) and proper moorsh (Z₃) characterizing different classes of peat transformation, i.e., weakly secondary transformed (W₁ = 0.41–0.50), medium secondary transformed (W₁ = 0.51–0.60), strongly secondary transformed (W₁ = 0.61–0.70), very strongly secondary transformed (W₁ = 0.71–0.80) and completely degraded (W₁ >0.80). The Brunauer-Emmet and Teller (BET) equation was used to analyse the experimental adsorption results and to calculate the specific surface area from both adsorption and desorption isothenns. Generally, the values of the specific surface area obtained from desorption isotherms were higher than those obtained from sorption curves. The shape of all curves was similar; i.e., all the curves belonged to the same class of isotherms according to the BET classification. The values of the correlation coefficient, R, indicated that the BET equation provided a good fit to the experimental data. For all studied samples the BET specific surface area estimated from the sorption isotherm ranged between 250 and 340 m² g^–1 and between 320-460 m² g^–1, if the desorption isotherms were used. The average BET specific surface area for the samples of the peaty moorsh (Z₁) and the proper moorsh (Z₃) were similar. The relationship between surface area and water holding capacity index W₁, characterising the state ofthe secondary transformation was found.