Abstract Based on the titration curves, the hydrogen-ion equilibria of α-chymotrypsin (EC 3.4.4.5), monoacetyl-α-chymotrypsin and diisopropylphosphoryl-α-chymotrypsin are identical within experimental error. This finding rules out various mechanisms which require the masking or the release of an ionizing residue for catalytic activity. No evidence could be found for the stable acylation of an imidazoyl moiety.