Autoxidation of acetophenones in butyric acid solution is thought to proceed through the enol forms. Enolization of these compounds was therefore separately studied, employing acetic acid- d as a solvent. Rates of deuteration—presumed to be identical with rates of enolization—were determined by means of NMR analyses; the nuclear protons do not exchange and served as the internal standard for methyl protons. Substituent effects may be expressed in terms of a Hammett rho-sigma relationship, with ϱ = −0.7 at 110 °C and using the σ + constants proposed by Brown and Okamoto. For acetophenone, the Arrhenius activation energy (65–120 °C) amounted to 16 kcal. The reactions were first order with respect to the ketones. Sodium acetate and pyridine showed a marked accelerating effect on H D exchange rates, which was proportional to the amount of base. Base-catalyzed contributions to rate constants could be represented by a Hammett-type equation, with ϱ = +0.7; however, σ constants led to a better fit than σ + substituent constants. Presumably, deuteration in the absence of base constitutes a true measure of enolization rates, i.e., it occurs through the enol, formed in a two-step process; base catalysis is attributable to reversible proton removal from the ketone.