Clostridium acetobutylicum cells were collected from chemostats which were run at pH 4.3 or 6.0 and which produced either acetone-butanol or acetate-butyrate; they were used to determine the level of enzymes involved either in solvent or in acid formation. The highest activity of phosphotransacetylase, phosphotransbutyrylase, acetate kinase, and butyrate kinase was found in cells which carried out an acetate-butyrate fermentation; these enzymes were present in solvent-producing cells at a level of about 10-50% as compared to acid-producing cells. Hydrogenase activity was detectable in approximately the same amounts in both cell types; however, in solvent-producing cells it was only measurable following a lag-period. Butyraldehyde and butanol dehydrogenases were found in small amounts exclusively in solvent-producing cells. It was demonstrated that the formation of acetone was initiated by the action of a coenzyme A-transferase which transferred coenzyme A from acetoacetyl-CoA to either acetate or butyrate. This coenzyme A-transferase as well as acetoacetate decarboxylase were hardly detectable in acid-producing cells, but reached high levels in solvent producing cells. Similar changes of the activity of the enzymes mentioned were observed when a batch culture was shifted from acid to solvent formation.