Rate constants have been determined for the nitrosation of methylurea (MU) in acid solution and also for the reverse reaction, the denitrosation of N-methyl-N-nitrosourea (MNU). The nitrosation reaction is essentially irreversible at the low acidities (0.01–0.4M-H₂SO₄) chosen for the experiments whereas the denitrosation reaction was examined at higher acidities (0.5–2.7M-H₂SO₄) in the presence of excess of hydrazine sulphate (a trap for free nitrous acid) when it is irreversible. For nitrosation the rate law, rate =k[HNO₂][MU][H⁺] was established and there was no catalysis by substantial concentrations of added potassium bromide or potassium thiocyanate. Similarly the rate law for denitrosation was found to be rate =k[MNU]h_A(where h_A is the acidity function used for the protonation of amides), and again there was no catalysis by added potassium bromide, potassium thiocyanate, or thiourea. The absence of nucleophilic catalysis in the nitrosation of amides had previously been noted and is a puzzling feature when comparison is made with the well established catalysis for amines. This is explained, together with the other observed results, by a detailed consideration of the individual kinetic steps involved in both reactions, and in particular by application of a limiting condition to both forward and reverse reactions in which the actual rate constant for the denitrosation process in an amide is very large compared with the proton transfer to the solvent from the intermediate.