In the solid state, 2-phenylazo-1,3-indandione, C1.5H10N2O2, exists as a keto-hydrazone tautomer 2-phenylhydrazono-1,3-indandione. The bond distance of one of the keto groups, is slightly elongated [1.230 (3) cf. 1.217 (2) angstrom] as a result of resonance. This resonance is also indicated by an averaging of the single and double bonds in the crystal structure. A strong intramolecular hydrogen bond is found between the elongated oxo group and the amine H atom.