The 1 : 1 charge-transfer complex of 9-anthraldehyde hydrazone with sulphur dioxide is not ionic in the ground state. Photolysis of the hydrazone in liquid sulphur dioxide or in liquid sulphur dioxide–carbon tetrachloride yielded 9-anthraldehyde azine (I) and nitrogen via N–N scission only. The quantum yield for formation of compound (I) varied with solvent and the concentrations of sulphur dioxide and oxygen. Quenching experiments indicated the intermediacy of a triplet charge-transfer complex between the hydrazone and sulphur dioxide. The rate constant for quenching of the complex, k_aca. 3·0 × 10⁸ l mol^–1s^–1, and the equilibrium constant for formation of the complex, K₃ca. 1·7 × 10² l mol^–1, were evaluated from experiments in the presence of air. Addition of oxygen caused a large increase in the quantum yield for formation of compound (I), suggesting that it is formed from an ionic intermediate via the triplet charge-transfer complex.