The mechanisms of the hydrolysis reactions of some N-nitrobenzenesulfonamides (YC6H4SO2NHNO2), N-nitrobenzamides (YC6H4CONHNO2) and N-methyl-N-nitrobenzamides (YC6H4CON(CH3)NO2) have been determined in aqueous sulfuric acid using the excess acidity method. Also studied were N-methyl-N-nitroacetamide and nitrourea, with N,N-dinitromethylamine for comparison. N-Nitrobenzenesulfonamides give either YC6H4SO2+ and NH2NO2 (electron-donating Y) or YC6H4SO2NH2 and NO2+ (electron-withdrawing Y) in A1 processes; the change in product is reflected in the different ρ+ values found for the two modes of cleavage. N-Nitrobenzamides behave similarly in strong acid, with an A1 reaction following presumed O-protonation, but in more moderate acid they exhibit a neutral water-catalysed hydrolysis mechanism, and in dilute acid the parent N-nitrobenzamides actually show hydroxide catalysis. N-Methyl-N-nitroacetamide shows only the neutral water-catalysed process. Nitrourea has an A1 acid-catalysed hydrolysis reaction in acid, analogous to the known B1 mechanism in base (also visible in dilute sulfuric acid), but has no water reaction; the pH-rate profile for the hydrolysis of this substance is here extended into the non-ideal acid region. N,N-Dinitromethylamine loses NO2+ in an A1 process following initial nitro-group protonation, giving N-nitromethylamine which is identifiable by its known hydrolysis rate. Activation parameters, m*m‡ slopes and ρ+ values given by the excess acidity analysis are shown to be compatible with the postulated mechanisms.