The paper is devoted to a study of the influence of the molecule structure of a number of o,o´-dihydroxyazo (DHA) compounds on their bactericidal ability toward sulfate-reducing bacteria (SRB) in the Postgate medium at small concentrations (5-20 mg/L). DHA compounds induce a decrease in number of bacterial cells, H 2 S production by SRB, and hydrogen permeation into carbon steel in the presence of SRB. The inhibition of steel corrosion in the bacterial medium by these substances is considered as well.