The influence of acetic acid (AcOH) and trifluoroacetic acid (TFA) on the hydrogenation of ethyl-4,4,4-trifluoroacetoacetate has been investigated by using Pt/Al2O3 modified by cinchonidine and O-methylcinchonidine. We have shown that the sometimes dramatic changes in enantioselectivity and rate cannot simply be interpreted by protonation of the alkaloid modifier. We propose a new three-step reaction pathway, involving interaction of the carboxylic acid with the reactant and the chiral modifier. The mechanism is supported by IR spectroscopic identification of cyclic TFA–modifier ion pairs. This new approach can rationalise the poorly understood role of acids in the enantioselective hydrogenation of activated ketones over cinchona-modified platinum metals.