Clostridium autoethanogenum produces ethanol from waste gases, but the biosynthetic pathway has been debated. Now, a combination of structural and biochemical data confirms that a key step in the ethanol biosynthesis pathway is acetate reduction by a tungsten-dependent aldehyde:ferredoxin oxido-reductase. This thermodynamically unfavorable reaction is counterbalanced by the coupling of ethanol synthesis with CO oxidation.