AbstractThioacetylacetone and its variously deuterated isotopomers have been investigated using electronic and vibrational spectroscopy combined with quantum chemical calculations. Thioacetylacetone is known for its photochromic properties, but the structures of the initial and final forms have been the subject of a long debate. Analysis of the IR spectra recorded in low‐temperature argon and xenon matrices, room‐temperature solutions, and in the gas phase has allowed us to establish the nature of the photochromic species and of its precursor. Similar to the case of another β‐thioxoketone, monothiodibenzoylmethane, the photoproduct has been assigned to the nonchelated SH exo‐rotamer of the (Z)‐enethiol tautomeric form, whereas the dominant ground‐state species corresponds to the chelated (Z)‐enol tautomeric form. Detailed vibrational assignments have been proposed for both forms based on quantum chemical calculations and polarization experiments. In the case of the chelated (Z)‐enol species prevailing in the ground state, a second‐order perturbative anharmonic analysis at the B3LYP/cc‐pVTZ level indicated strong anharmonic effects associated with the intramolecular hydrogen bond, leading to a shift of more than 600 cm−1 of the wavenumber of the OH‐stretching vibration. A small fraction of the SH endo‐rotameric chelated (Z)‐enethiol form was also detected under unperturbed conditions. The (Z)‐enethiol form can be converted into the (Z)‐enol form by irradiation at 290 nm.