To explain the adverse effect of SO2 activities the following mechanisms are proposed: 1) increased H+ concentration influences the physiology of microbes, changes their enzyme production, and the released enzymes are distant from optimum pH; 2) immissions act on the accumulation of carbonaceous substances associated with changes in C:N ratio; 3) toxic sulphur compounds break the structural, and thereby also functional integrity of microbes; they alter their enzymic outfit and inhibit the liberated enzymes.