A general synthetic method for oxosulfonium salts by oxidation of sulfonium salts with sodium perbenzoate (or sodiumm-chloroperbenzoate was developed. In case of the oxidation of aryldimethylsulfonium salts, the corresponding oxosulfonium salts (1e-h) were obtained in 64–91 % yields. Diphenymethylsulfonium and triphenylsulfonium salts were also oxidized with sodium perbenzoate to afford the corresponding oxosulfonium salts (1i and 1j) in 75 and 58% yields, respectively. Trialkylsulfonium salts such as trimethylsulfonium, dimethyloctylsulfonium, S-methylthiolanium, and S-methyl(pentamethylene)sulfonium salts, were also oxidized to the corresponding oxosulfonium salts (1a-d) in good yields. To clarify the reaction mechanisms, the oxidation of bicyclo[2.2.l]heptane-1-sulfonium salt (5) was investigated and found to afford oxosulfonium salt (6) in 50% yield. A reaction mechanism involving nucleophilic attack by perbenzoate anion on the cationic sulfur atom of sulfonium salt and giving an S-0 sulfurane intermediate is proposed.