Laboratory studies of nitrogen and water vapour adsorption on mucks originating from Lublin Polesie Region are presented. The Brunauer-Emmet and Teller (BET) equation was used to analyze the experimental results and to calculate the specific surface area from both nitrogen adsorption and water vapour sorption isotherms in relation to the degree of mucking (secondary transformation). The existence of relationship between surface area determined from water vapor desorption data and water holding capacity index W1 which characterizes the state of the secondary transformation of peat was proved. The relationships between the specific surface areas determined from water vapour and nitrogen adsorption data, and some important physical properties, eg ash content, bulk density and total porosity of the mucks studied were found. Both classification schemes of the secondary transformation of the peaty-muck soils, that according to Okruszko and W1 index according to Gawlik, should be used simultaneously. In order to characterize the state of muck transformation, simultaneous evaluation of specific surface area from adsorption of polar and non-polar adsorbates is advisable.