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An invisible, reversible catalytic reaction called enolization occurs consistently when carboxylic acid vapors contact metal oxide surfaces, a pathway widely invoked in mechanistic proposals for decarboxylative ketonization but not sufficiently examined experimentally. While the CO₂H group responsible for adsorption readily forms surface carboxylates, the weaker α-C–H acidity becomes evident only through reversible H/D exchange. The formation of an enolized surface carboxylate enables its subsequent condensation with a second carboxylate, a transformation widely regarded as the rate-determining step in the decarboxylative ketonization mechanism relevant to oxygen removal in biofuel upgrading. In our kinetic study, the rate of approaching equilibrium was measured for H/D isotopic exchange on alpha-carbon of isobutyric acid used in various concentrations in a vapor phase mixture with D2O as well as for reversed D/H exchange between alpha-deuterated isobutyric acid and H2O upon contact with monoclinic zirconia and anatase titania catalysts. Faster rate for H/D vs. D/H exchange points to alpha-deprotonation, i.e., enolization, as the rate determining step of the exchange mechanism. The intrinsic rate of enolization was deduced using McKay equation for equilibrium reactions. Kinetic activation parameters were obtained through temperature dependence of the rate constant for both exchange directions, H/D and D/H. KOH doping on ZrO2 changes the geometry of the transition state leading to higher rates of enolization and increasing H/D kinetic isotope effect from 1.4 to 5.8. The opposite effect of KOH doping is observed on anatase TiO2 – enolization rates are slightly decreased, kH/kD remains relatively constant at 2.6–2.8 indicating that the nature of basic centers on TiO2 is unaffected. These results confirm C–C coupling, not enolization, being the rate limiting step of the decarboxylative ketonization mechanism.