Determination of the formula of the labile coloured complex, formed by the reaction of thiosulphates with ferric salts, by the application of Job's method of continuous variation to the optical properties of the coloured complex has been reinvestigated due to the special nature of the decay curve. Three aqueous solutions of two ferric salts, one without acid and two others with defferent acid contents, have been used at two wave-lengths, 500 m\(\mu\) and 600 m\(\mu\) . The maximum absorption for the volume ratio of 1:1 of equimolecular solutions of ferric salt and thiosulphate without any acid confirms the formula Fe+SO3. The volume ratio changed to 1:2 and 1:3 according to the different acid contents. This has been explained as due to the difference between the effective concentration and the actual concentration of S2\(O^2_3\)- ion. When acidic solutions of ferric salts were used, the absorptions at zero-lime for wave­lengths 500 m\(\mu\) and 600 m\(\mu\). were not much different. But with solutions containing no acid, the di­fference was very high. The increase in absorption is case of aqueous solutions containing no acid at lower wave-length region is adcribed to the polynuclear hydrolysis product Fe(OH)2FE4+. Indication fur the existence of Fe(OH)2 FE2+(S2O3) has also been given.