[en] Singlet oxygen (¹O₂) could be generated in biological systems by endogenous and exogenous processes (e.g. enzymatic and chemical reactions, UV or visible light in the presence of a sensitizer). Numerous data show that proteins are the major targets of ¹O₂-induced damage in the living cells. In particular, reaction of ¹O₂ with thioether sulphur of methionine (Met) leads to the formation of persulphoxide >S⁽⁺⁾O-O^(-) which is in equilibrium with superoxide radical-anion (O₂^·-) and respective sulphur-centered-radical-cation >S^·+. In presented work, investigation the mechanisms of deprotonation and decarboxylation of the S^·+ - the irreversible processes, which competes with the formation of sulphoxide. Using thioethers dissevering by the number and positions of carboxylate groups it has been shown that efficiency of both decarboxylation and deprotonation could be influenced by various factors such as neighbouring group participation and environmental effects. The observed influence of carboxylate groups in β-position relative to the sulphur on the efficiency of decarboxylation suggests furthermore that they may also catalyze decarboxylation of α-positioned carboxylate in a manner similar to hydroxide anion