1(bis-p-Chlorophenyl)methylene-3-oxo-4,4-dimethyl-1,2-diazetidinium hydroxide, inner salt, C17H14Cl2N2O, crystallizes with orthorhombic symmetry, a = 9.95(1) Å, b = 28.73(2) Å, c = 11.30(1) Å, space group Pbca, Z = 8. The structure of this compound and its isostructural bromine analog were determined. The structure of the chloro-derivative was refined by full-matrix least-squares techniques using 1256 unique reflections to a final R of 0.099. The structure has a short intramolecular C—H … N bond (C … N distance = 2.891(11) Å). One of the phenyl rings is in the same plane as the 4-membered ring whereas the other ring lies in a plane almost perpendicular to this plane. The N—N distance in the heterocyclic ring is 1.415(10) Å, and the N—C distances in the ring are 1.535(11) and 1.377(11) Å. The molecular configuration is stabilized by both the short N … H interaction and delocalization between one of the phenyl groups and the diazetidinium ring.