A rapid titration procedure is recommended for the estimation of Na2S2O4 in commercial sodium dithionite (sodium hydrosulphite, sodium hyposulphite) and in aqueous solutions of this compound, using potassium hexacyanoferrate(III) as reagent and methylene blue as indicator. With the procedure described there is no interference of decomposition products of dithionite (sulphate, sulphide, sulphite, trithionate, thiosulphate), additives (sodium chloride, phosphates, soda, EDTA, urea), methanol, ethanol, acetone or groundwood. For a commercial sodium hydrosulphite containing 91% of Na2S2O4 the standard deviation is 0,3% absolute (degrees of freedom 9).