Abstract: A process for the oxidation of organosulphur compounds with hydrogen peroxide is provided. The process employs solid supported polyacids comprising tungsten, molybdenum and/or vanadium as catalysts. The supports are selected from compounds, preferably oxides, of Group IIa, IIb, IIIb, IVa and IVb elements, and strong base ion exchange resins. Certain embodiments of the process provide for the oxidation of sulphides to sulphoxides or sulphones, particularly for the oxidation of penicillins to penicillin sulphoxides. Other embodiments of the process provide for the selective oxidation of thiols to sulphoxides.