The diazotization of 1-naphthylamine has been studied at acidities ranging from 10−2 to 1.5 M. At low acidities nitrosation takes place via the unprotonated form of the amine, and the rate-controlling step is either the interaction with the nitrosating agent or, if high concentrations of a nucleophile are present, the loss of a proton from the protonated nitrosamine (the nucleophiles studied were Cl−, Br−, SCN−, and thiourea). At high acidities the protonated and free amines also react, the former of these reactions involving proton transfer to the solvent. In each case the proposed mechanism was supported by studying the reaction in D2O, which also allowed the isotopic effect of the solvent on the formation equilibria of the various nitrosating agents to be obtained.