The mechanism of the hydrolysis of acetylimidazole in aqueous perchloric, sulfuric, and hydrochloric acid mixtures has been determined. Benzoylimidazole was also studied in the latter two acids. The method of analyzing the available data, pseudo-first-order reaction rate constants as a function of acid concentration and, in one case, temperature, is the excess acidity method, here applied to the same reaction in the three different acid media, allowing their comparison. The reaction is not acid catalyzed; the rates decrease with increasing acidity. The substrate reacts in the form that is monoprotonated on the imidazole ring; it is 100% protonated at acidities much lower than those used here. Acetylimidazole is shown to become diprotonated at high acidity [Formula: see text], protonating on the carbonyl oxygen, but the diprotonated form is not reactive. The hydrolysis involves the reversible addition of one water molecule to the substrate to give a tetrahedral intermediate; at low acidities the decomposition of this hydrate is the rate-determining step, but as the acidity increases and the water activity decreases its formation becomes rate limiting. Hydroxide catalysis was also observed in dilute perchloric acid, but this is swamped by nucleophilic catalysis by the acid anion in HCl and H2SO4. Keywords: acylimidazoles, excess acidity, hydrolysis, protonation, tetrahedral intermediate.