Figure schematically represents the different reaction pathways we computed to form the thio-(hydro)peroxy acid. Among the four spiral arms the oxidation of the thiol moiety with the singlet oxygen, shown in pathway (a), turned out to be the most efficient mechanism with its considerably low barrier (7.0 kcal/mol) when attacking the sulfur atom directly. The alternative pathway in Figure (b), which also included the singlet oxygen, exhibits a higher barrier (17.9 kcal/mol) upon the attack to the S-H bond. It can therefore be stated that fetching the electrons directly from the lone pair of the sulfur atom is a more favourable process.