The absorbance extremum method introduced to resolve the rate constant–reaction step ambiguity problem in two step consecutive reactions is applied to cases where the rate constant in each step is pseudo first order and dependent on catalyst concentration. In particular, acid and base catalyzed reactions of substrates in aqueous solution are examined. Resultant simultaneous pH–rate profile functions are treated in the context of the ambiguity problem and a detailed account of the possible interactions between pH–rate profiles is also given. The merits of this method include its general scope, its applicability to reactions for which it may be experimentally difficult to resolve the ambiguity assignment by other means, and its use of rate data acquired from original kinetic traces without requiring additional information. The theoretical analysis is verified and tested experimentally for the hydration of fluorenylideneketene in dilute aqueous perchloric acid solutions.