Thiourea is shown to be a very efficient catalyst in the nitrosation of morpholine in acid solution. Compared with other known catalysts, the order of efficiency is SC(NH₂)₂ > SCN^– > Br^– in the ratio 4 200 : 240 : 1. The kinetic data are interpreted in terms of equilibrium formation of the NO–S ion from thiourea which acts directly as a nitrosating agent. The overall efficiency of thiourea as a catalyst arises from the large equilibrium constant for the formation of this ion rather than from the rate constant (k₂) for attack by the ion on morpholine. Values of k₂ for the series SC(NH₂)₂, SCN^–, and Br^– at 31° increase from 7 × 10⁶ to 2.9 × 10⁷ and 4.8 × 10⁷ l mol^–1 s^–1, respectively. Similarly the diazotisation of aniline proceeds via NO–S in the presence of added thiourea. Again the k₂ values were determined at 0° for the series SC(NH₂)₂, SCN^–, Br^– as 3.7 × 10⁵, 1.1 × 10⁷, and 1.6 × 10⁹ l mol^–1 s^–1. For both SC(NH₂)₂ and SCN^– reactions, the rate constant is well below that of the diffusion-controlled limit (cf. Br^–); the measured activation energies also support this conclusion. It is suggested that thiourea and possibly other sulphur-containing species act as excellent general catalysts for nitrosation and diazotisation reactions generally. Cysteine showed negligible catalytic activity whilst S-methylcysteine gave some indication of catalysis in the diazotisation of aniline, but the effect was small.