1,3,3,5-Tetraarylpentane-1,5-diones 2 and 5 react with tetraphosphodecasulfide in xylene at elevated temperature to give the corresponding 2,4,4,6-tetraaryl-4H-thiopyrans 4 and 7. The mechanism of this transformation involves a fast initial step of dehydration of 2 and 5 to 2,4,4,6-tetraaryl-4H-pyrans 3, followed by a slower transformation to the 4H-thiopyran analogues 4. Using 2D NMR techniques, a complete assignment of the ¹H and ¹³C NMR spectra of the 1-oxide and 1,1-dioxide analogues of 4(1, X = SO and SO₂) as well as of compounds 3c, j, I, m, 4a–m, 6(the pyran analogue of 7) and 7 has been carried out. Compounds 4 and 7 undergo a reversible photochemical colour change after UV illumination. The maxima of the new absorption bands are situated at 540–660 nm. The non-exponential time dependence of the photodecolouration of 4a in the solid state is analysed in terms of dispersive first-order reaction kinetics. Dioxygen accelerates the decolouration process; the half-life at 299 K lies between 6600 s in air and several days in a vacuum (10^–3 Pa). The trapping of photoproducts 10a, 10e and 11a enables us to postulate the whole photolysis sequence.