Trithiocyanuric acid (C3H3N3S3) monomers were studied using FTIR spectroscopy combined with the matrix isolation technique. The matrix-isolated compound adopted only the trithione tautomeric form, as revealed by its IR spectra. Upon UV irradiation (lambda > 270 nm), the trithiol tautomeric form of trithiocyanuric acid (trimercaptotriazine) was generated. This is the first observation of an intramolecular triple trithione -> trithiol photoinduced proton transfer. The substrate and the product of the photoreaction were identified by comparison of their IR spectra with the spectra theoretically calculated (at the DFT(B3LYP)/6-31++G(d,p) level) for the trithione D-3h and trithiol C-3h structures, respectively. The IR absorption bands observed in the experimental spectra were assigned to the calculated normal modes. The theoretical normal modes were analyzed in terms of symmetry-adapted internal coordinates. Consistent sets of symmetry coordinates applicable for the description of in- and out-of-plane movements of six-membered rings with D-3h and C-3h symmetry were defined for the first time.