Although many 3,4-dihydro-4-oxo and -imido-derivatives of 1,2,3-benzotriazine are known, 1,2,3-benzotriazine itself was previously unknown, only very few of its aromatic derivatives had been reported, and no general methods for their synthesis had been described. 1,2,3-Benzotriazine, some simple 4-alkyl and aryl derivatives, and a pyrido[1,2,3]triazine are now described, together with four general syntheses of the ring system. These involve (i) oxidation of the hydrazones of o-azidophenyl ketones and thermal cyclisation of the resulting o-azidophenyldiazoalkanes, (ii) oxidative cyclisation of the hydrazones of o-aminophenyl ketones, (iii) oxidation of 1- and 2-aminoindazoles, and (iv) oxidation of N-aminoquinazolin-2-ones. Methods (iii) and (iv) involve heterocyclic ring expansions, the latter with loss of carbon monoxide. 1,2,3-Benzotriazines are susceptible to nucleophilic addition to the 3,4-bond, and the high reactivity of benzotriazine in this reaction explains the failure of earlier attempts to synthesise it. Preparation of the azido-hydrazones required for method (i) from azido-ketones in the standard way led to an unusual but useful synthesis of indazoles.