AssmcrThe rate of diazotisation of aniline, 0-, m-, andp-, chloroaniline, in 0.1 rnol/l sulphuric acid, was determined at different temperatures. The values of the specific reaction rate, free energy, enthalpy, entropy, and energy, of activation, for the rate-determining reaction, viz, the nitrosation of the free amine by nitrosyl sulphuric acid, were calculated and compared. Relations were drawn between each of the specific reaction rate, and the free energy of activation, for the rate-determining reaction, and base strength. The effect of the chlorine atom, in the different positions of the benzene nucleus, of the chloroanilines, on the reaction rate, is discussed.