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Deprotection of 1,3-dithianes and 1,3-dithiolanes of aromatic and aliphatic aldehydes and ketones have been performed under irradiation in presence of a variety of sensitizers. In these reactions, the disulfide is converted into de carbonyl group; however, the operating mechanism as well as the source of oxygen in these reactions is a matter of controversy.[i] We have studied the photorelease of a number of 1,3-dithianes carbonyl protecting group by photosensitized induced oxidation using thiapyrylium cation (Spy+) as sensitizer (eq. 1). The preparative aspect of the reaction has been studied and we have found that the presence of oxygen is essential for a good conversion. In order to establish a complete mechanistic picture of this valuable photoreaction, transient absorption spectroscopy together with quantum chemical exploration at a BHHLYP/6-311++G(d,p) level of theory have been performed. A broad absorption band at 500 nm, assigned to the 1,3-dithiane radical cation was found by laser excitation of the thiapyrylium salt. This intermediate follows a first order kinetic decay which is not affected by the presence of molecular oxygen or water. The energy surface for the Carbon-Sulfur bond fragmentation was considered, the distonic radical cation 1 was found as intermediate. Finally, the reactivity between radical cation 1 and water, and the reaction between radical cation 2 and molecular oxygen or superoxide anion will be discussed (Scheme 1). [i](a)Kamata, M.; Otogawa, H.; Hasegawa, E. Tetrahedron Lett. 1991, 32, 7421. (b) Kamata, M.; Kato, Y.; Hasegawa, E. Tetrahedron Lett.1991, 32, 4349. (c) Epling, G. A.; Wang, Q. Tetrahedron Lett. 1992, 33, 5909. (d) Fasanim E.; Freccero, M.; Melle, M.; Albini, A. Tetrahedron, 1997, 53, 2219.