Trithiocyanuric acid (C(3)H(3)N(3)S(3)) 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(3)(h) and trithiol C(3)(h) 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(3)(h) and C(3)(h) symmetry were defined for the first time.