The enzyme acyl-phosphate-hexose phosphotransferase from Aerobacter aerogenes was purified to electrophoretic homogeneity. The molecular weight of the enzyme as determined on Sephadex gels is 150 000. The enzyme possesses potent phosphotransferase and phosphohydrolase activities. Initial rate kinetics were used to investigate the mechanism of acyl-phosphate-hexose phosphotransferase. These studies, which involved a number of different phosphoryl donors and substrate analogues, suggest that the kinetic mechanism is of the rapid equilibrium random Bi Bi type. A number of other enzymes that exhibit both transferase and hydrolase activities involve obligatory covalent enzyme-substrate intermediates in their mechanisms of action.