Equations have been derived for the single substrate case to include the effects of the ionization of a group in the substrate molecule upon the over-all rate of an enzymatic reaction. These equations have been described in terms of changes in the maximum velocity and Michaelis constant of the reaction. It is shown that there are several distinguishable kinetic cases even when the effects of substrate ionization are complicated by those of groups in the enzymatically active site. These cases may be distinguished by differences in plots of kinetic constants as a function of pH for the forward and reverse reaction. Provided that enough data are available and that the assumptions made in the derivations are correct, it is possible to tell whether (1) one of both ionic forms of the substrate are utilizable or (2) if only one form is utilizable, whether the other form is a good or poor competitive inhibitor.