Diazotisation and aso=coupling reactions have been studied and the results are reported. A kinetic study of the acid catalysed diazotisation of various aniline derivatives has been made. The substrates considered were 2,4-dinitro-aniline, 4-nitroaniline, sulphanilaraide (aniline-4-sulphonamide) and sulphanilic acid (aniline-4-sulphonic acid). A new pK(_a) value of l.4 is reported for sulphanilamide. A study of the diazotisation of sulphanilic acid and sulphanilamide in the presence of Cl¯, Br¯, SCN¯, and SC(NH(_2))(_2) shows that the general catalytic trend Cl¯ < Br¯, SCN¯ < SC(NH(_2))(_2) is followed; whereas for the corresporaing nitrosating agents the general reactivity trend NOCl > NOBr >; NOSCN >NOS(^+)C(NH(_2))(_2) is followed. No catalysis was observed in the case of 2,4=dinitroaniline and this has-been interpreted in terms of a rapid reversible nitrosation in which the rate of the denitrosation step is greater than the rate of decomposition of the intermediate nitrosammonium ion. The bimolecular rate constants obtained for sulphanilic acid and sulphanilamide approach the diffusion controlled limit for this process in the case of NOCl and NOBr, Also reported is a study of the diazotisation of two heteroaromatic amines: 2-amino-5-nitrothiazole and 3-amino-l,2,4-triazoleo Catalysis by acid and the nucleophiles Cl¯ and SCN¯ has been observed. The catalytic and reactivity trends described above have been noted for reaction of these heteroaroraaticso The Pk(_a) value for 2-amino—5-nitrothiazole has-been determined and agrees well with the literature values. In addition to the above studies of diazotisation mechanisms, results are reported of a study of the leaving abilities of various electrofugal leaving groups X during ipso=coupling of 4=nitroaniline with 4-X,N- dimethylanilineso The sequence of leaving abilities has been determined by product yield studies. Rate constants for attack of ArN(^+)(_2) have also been determined. It is suggested that a substrate molecule (X-Ar-NMe(_2)) acts as a base in removing the group X from the Wheland intermediate.