The kinetics of hydrolysis of 4-(p′-methoxyphenylazo)pyridine,1, and its 3-isomer,2, have been studied in moderately concentrated sulfuric acid media at 25 °C. In all the acid solutions investigated,1reacted faster than2; rate differences between the two compounds varied from ca. 1000-fold in the dilute region of acidity to ca. 250-fold in the more concentrated acid solutions. The observed first-order rate constants,kψ, for both substrates exhibit a maximum, at ca. 42% H2SO4and 47% H2SO4for1and2respectively. Activation parameters have also been determined. The pKavalues for the second protonation equilibria of1and2have been evaluated and structures of the diprotonated species are discussed. Hydrolysis is shown to occur from the diprotonated substrates and two main mechanisms are operative. The first is an A-2 type mechanism, which involves rate-limiting attack of H2O on the aryl carbon center giving delocalized transition states and intermediates in which the pyridinium and azonium functions are involved in charge delocalization. Subsequent transfer of a proton and detachment of the leaving group are fast processes. In the second A-SE2 type mechanism, nucleophilic attack and transfer of the proton are fast steps preceding the slow general acid catalyzed separation of the leaving group. The difference in reactivity of the two compounds is attributed to differences in extent of charge delocalization in the transition states of the reactions: for1both the pyridinium and protonated azonium functions are involved whereas for2only the azonium function participates in charge delocalization.