A large quantity of sulfite interferes the formation of white colloidal turbidity in mercuric bromide test for the determination of extremely small quantity of thiosulfate. Disappearance of turbidity does not be due to relative quantity of sulfite to thiosulfate, but is caused by the formation of complex compound of sulfite and mercuric bromide reagent. The formation of the stable compounds of sulfite and mercuric bromide reagent was ascertained by the measurements of spectral absorptions of mixed solutions of sodium sulfite and mercuric bromide reagent at various ratios. Absorbancy at 232 mμ indicates maximum at 1: 1 (molar ratio) of sodium sulfite to mercuric bromide, with continuous variation method. With molar ratio method, absorbancy curve at 232 mμ shows the two break points at molar ratios 1: 1 and 1: 2 of mercuric bromide to sodium sulfite, indicating the presence of 1: 1 and 1: 2 compounds.