Judged by properties observed during the purification and based on the sequence of the first 25 amino acids, the enzyme from Clostridium formicoaceticum catalysing the reversible reduction of non-activated carboxylic acids to aldehydes at the expense of reduced viologens, is astonishingly different from that found by us in C. thermoaceticum. According to native and SDS gel electrophoresis the reductase is nearly homogeneous after only 26-fold purification. The specificity for various substrates and artificial electron carriers is also broad, but V of the purified aldehyde dehydrogenase activity (54 U/mg enzyme for butanal) is about 1 order of magnitude lower than that of the enzyme from C. thermoaceticum. The reductase is a dimer of two identical subunits with an Mr of 67,000 each. Increased enzyme concentrations seem to lead to higher oligomers. Per dimer 11 +/- 1 iron, 16 +/- 1 acid labile sulphur, 1.4 tungsten and after permanganate oxidation 1.6 mol pterin-6-carboxylic acid have been found.