ALTHOUGH the reaction of selenious acid with potassium permanganate has been used in estimating selenium1–3 it has not been investigated in detail. According to Charlot4 selenious acid reduces permanganate to manganese dioxide, which separates from the solution. But it is observed that when potassium permanganate is added to a large excess of selenious acid solution, the pink colour due to permanganate is discharged and the formation of precipitates of MnO2 does not take place. If selenious acid is not in large excess, only the precipitates of MnO2 appear shortly after the addition of permanganate. This suggests that selenious acid first reduces permanganate ion to manganous ion which reacts with permanganate to form manganese dioxide. In the present investigation the reaction between selenious acid and potassium permanganate has been studied by physico-chemical methods.