The products (a benzoate ester or anN,N-dimethylbenzamide) have been accurately determined for the hydrolysis of theO-ethyl-N,N-dimethylbenzimidatonium ion (1) and its 4-nitro derivative (2) in dilute and moderately concentrated H2SO4solutions. In dilute acid the ratio of amide:ester is very small (0.0005–0.001). It is proposed that the ester product arises from the zwitterionic form of the tetrahedral intermediate of the hydrolysis. The small amount of amide is due to an SN2 hydrolysis reaction and to the non-catalyzed breakdown of the neutral form of the tetrahedral intermediate, the relative contributions of the two being unknown. Although ester remains the major product throughout, at some intermediate acid concentration (20% H2SO4for1and 35% H2SO4for2) a sharp increase in the amide:ester ratio is observed, followed by a levelling-off. This change is shown to be associated with tetrahedral intermediate partitioning, and not the SN2 reaction. It is argued that the strong acid product ratio represents the breakdown of the tetrahedral intermediate via cationic transition States. Two kinetically equivalent reactions are involved, the breakdown of theN-protonated tetrahedral intermediate producing a protonated ester and amine and the H+-catalyzed breakdown of the neutral tetrahedral intermediate producing a protonated amide and alcohol. Analogous pathways are also observed for the decomposition in acid of the amide acetals ArC(OMe)2NMe2, and the two systems are compared. A similar effect of the aromatic substituent on the partitioning ratio is noted, but the tetrahedral intermediate exhibits a greater tendency for amine expulsion. A kinetic analysis produces the estimate of 1011 s−1for the rate constant for the breakdown of the zwitterionic form of the tetrahedral intermediate. This very large rate explains why its decomposition remains important even in concentrated acids, where the amount of tetrahedral intermediate which actually exists as zwitterion must be extremely low.