Surface geometric heterogeneity of peat-muck soils in various phases of the mucking process was investigated. The soil material represented two kinds of muck, namely the peaty muck (Z₁) and the proper muck (Z₃) soils. The soils were characterized by the water holding capacity index, W₁. ranging from 0.44 to 0.82. This index describes the stale of the mucking process. The adsorption of water vapour and nitrogen data were used to evaluate the surface fractal dimension (D) from the Frenkela-Hilla-Halscya (FHH) isotherm equation. For all studied samples the surface fractal dimension obtained from adsorption of nitrogen ranged between 2.25 and 2.59. whereas estimated from water vapour adsorption – ranged between 2.58 and 2.75. The relationship between D and the water holding capacity index W₁. porosity, bulk density. content of the organic carbon and the sum of humic and fulvic acids were found. The fractal dimension D(N₂) of the Z₁ soils, i.e., weakly secondary transformed (W₁<0.6) peat, were close to 2.49, whereas the values of D(N₂) for Z₃ soils, i.e., for strongly secondary transformed soils (W₁>0.6) were close to 2.37. The peat muck soils Z₃ exhibited lower fractal dimension D(N₂) and lower total porosity than proper peat-muck soils Z1. The fractal dimensions obtained from nitrogen adsorption data decreased significantly with an increase of organic carbon content and increased with the amount of humic and fulvic acids. In general, the main sources or the energetic and geometric heterogeneity of the solids are the complex chemical composition and irregularities of the mineral skeleton. However, in the case of peat-muck soils, the porosity seems to be the main factor determining their geometric heterogeneity.