Infrared (IR) and electronic spectroscopy studies were carried out for monothiodibenzoylmethane isolated in rare gas matrices, low-temperature glasses, polymer films, KBr tablets (IR) and liquid solutions. This molecule is known to undergo light-induced conversion, but the nature of the product has long been a subject of discussion. Comparison of the experimental IR spectra with theoretical predictions (B3LYP/cc-pVDZ) obtained for various possible structures shows that the photoproduct corresponds to a non-chelated -SH exo-rotamer of the (Z)-enethiol tautomeric form of monothiodibenzoylmethane. This conclusion is consistent with the experimentally determined energy of the activation barrier for the reverse ground-state process that brings the molecule back to the initial enolic, intramolecularly hydrogen-bonded structure.