Abstract Adenosine diphosphate and adenosine diphosphate ribose inhibit the yeast alcohol dehydrogenase-catalyzed oxidation of ethanol. In both cases, the inhibition observed was competitive with respect to nicotinamide adenine dinucleotide (NAD+). Adenosine diphosphate ribose is more effectively bound to the enzyme than is adenosine diphosphate, and this greater binding may reflect an interaction between adenosine diphosphate ribose and the hydrophobic region of the enzyme. The free energy of binding of adenosine diphosphate ribose and N 1 -methylnicotinamide chloride to yeast alcohol dehydrogenase was calculated from inhibitor constants. The sum of these negative free energies is larger than the negative free energy change of binding of the coenzyme, NAD +.