Aerobacter aerogenes, Aerobacillus polymyxa, and Aeromonas hydrophila, representatives of the three genera characterized by a butanediol fermentation, can all oxidize 2,3-butanediol under aerobic conditions. The configuration of the 2,3-butanediol has considerable bearing on its decomposability: Aerobacter aerogenes is inactive on the l-isomer, but attacks both meso- and d-isomers; Aeromonas hydrophila attacks the meso-isomer but not the l- and probably not the d-isomer; Aerobacillus polymyxa can oxidize both l- and meso-2,3-butanediol, but the rate with the former is many times greater than with the latter. Aerobacter aerogenes oxidizes both 2,3-butanediol and acetoin to carbon dioxide and water, a large part of the substrate being simultaneously assimilated. The other two organisms oxidize 2,3-butanediol to acetoin, but can further oxidize the acetoin thus formed only very slowly, if at all. Both Aerobacter aerogenes and Aerobacillus polymyxa are unable to attack 1,3-butanediol, 2-methyl-1,2-propanediol and 1,2-ethancdiol. However they can oxidize 1,2-propanediol to acetol.