The dehydrogenase activity (an index of the total soil biological activity), sulphur content, pH and Corg were determined in technogenic soils of the former (1954-1994) sulphur mines in Ukraine (open pit and underground sulphur melting, Yavoriv and Nemyriv, respectively). The soils were neither managed nor reclaimed, and underwent natural self-restoration processes. Soils of former open pit sulphur mine showed Corg of 0.07-1.29%, pH of 7-7.9 and a high SO4-S content (1.7-14.7 g kg-1). Dehydrogenase activity was 2-3 fold lower than in the control forest podzolic soil. Sulphur mining by underground melting resulted in strong soil acidification (pH of 1.5-4.5) and a drastic lost of soil dehydrogenase activity, despite the relatively low Stot content (0.02-0.2 g kg-1). In upper horizons dehydrogenase activity decreased up to 28 fold as compared to the forest podzolic soil. In soil located 40 m from the boring well, with a high Corg of 8-9% (versus 0.07-1.11% in other soils that area) the decrease in the activity was not as heavy (11-fold) despite a low pH (1.5-1.8) and higher Stot (2.1-3.3 g kg-1).