The H/D isotope effect on the catalytic production of hydrogen from water using either Cr2+ or (CH3)2COH radicals as reductants was studied. The catalysts used were gold sols stabilized by either citrate or poly(vinyl sulfate). Similar isotope effects (in the range of 5-6.7) were measured for the various reductants and stabilizers. Comparison of this isotope effect to the one measured for the production of hydrogen on gold cathodes leads to the conclusion that similar mechanisms operate in all three systems. It was also observed that hydrogen abstraction from the relatively stable supports, when adsorbed on the colloid, occurs. This leads to the conclusion that hydrogen atoms adsorbed on the particles can either proceed to produced hydrogen molecules by the catalytic reaction or react via other competing pathways.