The rate of diazotisation of 2-aminothiazole was measured in 65 to 75% (w/w) aqueous H,SO,. The kinetic solvent isotope effect of the diazotisation in 72% D2S04/D20 was determined (k,/k, = 5.8 i 0.2). These data are consistent with a mechanism in which the 2-aminothiazole, protonated at the ring N-atom, but not at the NH, group, is attacked by the NO+ ion. The reaction does not go to completion, but to an equilibrium. The equilibrium concentration of the diazonium ion was determined in 30, 50, 70, and 90% ( w / w ) H,SO, at three initial concentrations of reagents (0.001,0.01, and 0.1~). The final concentrations of the reagents and the diazonium ion are consistent with a reversible process. This is the first diazotisation for which quantitative evidence for equilibration has been found. In the very large range of acidities used (Ifo = -1.73 to -9.01), it was not possible to calculate meaningful equilibrium constants that are independent of the acidity.Introduction. -Diazotisation of five-and six-membered heteroaromatic amines has been investigated for many years, according to Butler's 1975 review [l]