The specific reaction rates for the chloride-catalysed diazotisation of aniline and p-toluidine were combined with equilibrium data on the proton ionisation of the anilinium and p-toluidinium ions, and on the hydrolysis of nitrosyl chloride. The specific reaction rates were calculated for the N-nitrosation of the free amines, which is the rate-determining stage in the diazotisation of the amines, under the present conditions. A G * , A H * , A S * , and E were determined for this rate-determining stage. These values were compared with the corresponding values for p-chloroaniline. The influence of the substituents on the rates of the N-nitrosation of these amines was discussed.