Three key enzymes of the Clostridium acetobutylicum ATCC 824 metabolism, namely thiolase, phosphotransbutyrylase (PTB), and CoA transferase, were purified to homogeneity. Each of these enzymes catalyzes the first reaction after a branch-point in the fermentative pathway, and may therefore be an important point in the regulation of the fermentation. A number of physical and kinetic properties, namely native and subunit molecular weight, K\sbm values, kinetic binding mechanism, optimum pH, substrate specificity, and effect of cofactors and metabolites on enzyme activity were determined with the objective of identifying important regulatory factors for each enzyme. Thiolase is inhibited by micromolar levels of CoASH in the condensation direction, and this may be the most important factor in modulating the net rate of condensation of acetyl CoA. The enzyme is relatively insensitive to change in pH within the physiologic range. Studies of thiolase specific activity under various types of continuous fermentations show that regulation of this enzyme at both the genetic and enzyme level is important. PTB is sensitive to pH change in the butyryl phosphate-forming direction, within the physiologic range of pH 5.5 to 7; relative activity decreases with decreasing pH to about 5% at pH 6. This indicates that change in internal pH may be one important factor in the regulation of the enzyme. The enzyme can use a number of substrates in addition to butyryl CoA, but has the highest relative activity with butyryl CoA, isovaleryl CoA and valeryl CoA. CoA transferase is very unstable in vitro unless high concentrations of both glycerol (20%) and ammonium sulfate (0.5 M) are present; virtually all activity is lost within an hour without either stabilizer. The K\sbm values for butyrate and acetate of 660 mM and 1200 mM, respectively, are high relative to the intracellular concentrations of these species; consequently, in vivo enzyme activity is expected to be sensitive to changes in those concentrations. In addition to the carboxylic acids listed above, this CoA transferase is able to activate propionate, valerate, isobutyrate, and crotonate. The enzyme activity is relatively independent of pH, but is inhibited by physiologic levels of acetone and butanol, suggesting a feedback inhibition mechanism.