Abstract In the dioldehydrase reaction, L(+) propanediol reacts at a faster rate than the D(−)-isomer in competitive reactions, but when the diols are run independently the rate of the D(−)-isomer is 2.3 times greater than that of the L(+)-isomer. It is assumed that this reversal is due to a difference in the Michaelis-Menten constants. A ratio of K mD(−)/K mL(+) = 3.2 ± 0.3 at 25° is calculated from a series of competitive reactions. It is concluded that the binding site is such that while both isomers are accomodated readily, the D(−)-isomer is hindered to some degree in its approach to the binding site. However, once the enzyme-substrate complex is formed, the D(−)-isomer is in a more favorable configuration for further reaction.