Sodium sulfide or sodium hydrosulfide is oxidized to form sodium polysulfide and sodium hydroxide which may be further oxidized to sodium thiosulfate and sodium hydroxide. The oxidant is air, oxygen or oxygen in mixture with other gases, the reductant is aqueous sodium sulfide or sodium hydrosulfide and the reaction is carried out with both reactants in non-flooded contact with a solid catalyst material, of which a typical example is a particulate carbon treated with polytetrafluoroethylene (PTFE). The reaction is typically conducted at atmospheric pressure and without the application of heat, with the oxidant and the reductant being in interfacial contact with each other, but only, insofar as possible, at sites where both are simultaneously in contact with a catalyst which is prevented from being flooded by either the oxidant or reductant.