Abstract: ~(35)S-labelled intermediates of two sulphureous pesticides, palathion and fenitrothion, were prepared from isotopic exchange between ~(35)Ssulphate and two intermediates (O, O, -dimethyl-S-disulphophosphate and O, O, -dimethyl-S-phosphorochloridothionate) with the existence of element sulphur, Then, ~(35)S-palathion and ~(35)S-fenitrothion were synthesized.