The deuterium enrichment of dihydroxyacetone obtained from the aldose-ketose isomerization of D,L-glyceraldehyde in D2O at 25 degreesC was determined by H-1 NMR spectroscopy from the integrated areas of the signals for the alpha -CH2 and alpha -CHD groups of the product. One mole equivalent of deuterium is incorporated into the product when the isomerization is carried out in 150 mM pyrophosphate buffer at pD 8.4, but only 0.6 mol equiv of deuterium is incorporated into the product of isomerization in the presence of 0.01 M deuterioxide ion, so that 40% of the latter isomerization reaction proceeds by the intramolecular transfer of hydride ion. Several pathways were identified for catalysis of the isomerization of glyceraldehyde to give dihydroxyacetone. The isomerization with hydride transfer is strongly catalyzed by added Zn2+. Deprotonation of glyceraldehyde is rate-determining for isomerization with proton transfer, and this proton-transfer reaction is catalyzed by Bronsted bases. Proton transfer also occurs by a termolecular pathway with catalysis by the combined action of Bronsted bases and Zn2+. These results show that there is no Large advantage to the spontaneous isomerization of glyceraldehyde in alkaline solution with either proton or hydride transfer, and that effective catalytic pathways exist to stabilize the transition states for both of these reactions in water. The existence of separate enzymes that catalyze the isomerization of sugars with hydride transfer and the isomerization of sugar phosphates with proton transfer is proposed to be a consequence of the lack of any large advantage to reaction by either of these pathways for the corresponding nonenzymatic isomerization in water.