The electrode processes occuring in the Fe(II)-S2O32- system on mercury electrode were investigated by cyclic voltammetry, chronopotentiometry, polarography and pulse polarography. Electrocatalytic thiosulphate ion decomposition was observed resulting in FeS formation. Formation of the solid FeS phase on mercury surface lowers the Fe(II) reduction overpotential and makes the wetting of iron by mercury impossible. Consequently, iron is deposited on the electrode surface. It was also found that the FeS formation efficiency in the Fe(II)-S2O32- system is much higher than in the Fe(II)-SCN- system. The high amount of FeS formed in the vicinity of the mercury electrode in solution of high thiosulphate ion concentration inhibits the iron deposition on mercury.