The invention relates to a method for producing sodium dithionite comprising the steps (a) production of a synthesis batch consisting of sodium formate, sulphur dioxide and an alkaline sodium compound in an aqueous methanol solvent, which forms a sodium dithionite mother liquor, (b) separation of the sodium dithionite from the sodium dithionite mother liquor, leaving a residual mother liquor, (c) separation of methanol from the residual mother liquor, (d) separation of the residual mother liquor into a first sub-stream that is rich in thiosulphate and a second sub-stream that is devoid of thiosulphate, but contains formate by means of a nanofiltration, (e) concentration of sodium formate in the second sub-stream and (f) return of the concentrated second sub-stream to step (a).