The structure of 5-( 1 -aminoisoindolerdn-3-ylideneamino)-1,3,4-thiadiazole-2(3 H)-thione was studied by X-ray diffraction analysis and UV and IR spectroscopy. In the crystal, the molecule of the title compound is planar in spite of the substantially shortened intramolecular N...S contact. The factors that determine the structure of the heterocycle, namely, the nature of the N...S interaction, conjugation effects, and the tautomerism, were analyzed based on quantum-chemical calculations performed by the AM1 method_