Four hydrogen-producing, aerotolerant, anaerobic bacterial strains isolated from chlorinated solvent-contaminated groundwater were characterized using a polyphasic approach. Three of the strains, designated BL-18, BL-19 and BL-20T, were found to be identical in 16S rRNA gene sequences and in phenotypic properties. Cells of these strains are Gram-positive-staining, spore-forming, motile rods with peritrichous flagella. Growth occurred at 15–40 °C, pH 5.0–10.0 and at NaCl concentrations up to 5 % (w/v). Acid was produced in fermentation of cellobiose, fructose, galactose (weak), glucose, maltose and salicin. Products of fermentation in PYG medium were acetate, butyrate, ethanol, formate, carbon dioxide and hydrogen. Dominant cellular fatty acids when grown in PYG medium were C13 : 0 iso, C16 : 0, C13 : 0 anteiso, C15 : 0 iso and C15 : 0 anteiso. The genomic DNA G+C content was 30.4 mol%. These isolates can be differentiated from their closest phylogenetic relative, the cluster I Clostridium species Clostridium frigidicarnis (97.2 % similar to the type strain in 16S rRNA gene sequence), on the basis of phenotypic and chemotaxonomic properties. The other strain characterized in this study, BL-28T, was Gram-positive-staining with spore-forming, rod-shaped cells. Growth occurred at 15–46 °C, pH 6.0–8.5 and at NaCl concentrations up to 3 % (w/v). Acid was produced from cellobiose, dextran, fructose (weak), glucose, maltose, salicin and trehalose. End products of PYG fermentation included acetate, butyrate, pyruvate, carbon dioxide and hydrogen. Dominant cellular fatty acids from cells grown in PYG medium at 30 °C were C14 : 0, C14 : 0 dimethyl aldehyde, C16 : 0 and C12 : 0. The DNA G+C content was 28.5 mol%. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain BL-28T falls within cluster I of the genus Clostridium, but with ≤95.2 % identity with previously described species. On the basis of results presented here, strains BL-20T (=NRRL B-51348T =DSM 21757T) and BL-28T (=NRRL B-51352T =DSM 21758T) are proposed as the type strains of novel species of the genus Clostridium with the names Clostridium hydrogeniformans sp. nov. and Clostridium cavendishii sp. nov., respectively.