Physical Chemistry Section, Jadavpur University, Calcutta-32. Manuscript received 30 July 1974 accepted 30 December 1974 Neutralized phenyl phospbonous acid is oxidised by alkaline hexacyanoferrate (III) in the presence of osmium (VIII). The rate of disappearance of hexacyanoferrate (Ill) is found to be independent of the initial concentration of oxidant but directly proportional to the concentrations of reducing substrate, hydroxyl ion and osmium (VIII). The results indicate that the oxidation of the subtrate proceeds via the formation of a complex between reducing substrate (VI) and osmium (VIII) which then reacts with hydroxyl ion followed by a fast reaction between osmium and hexacyanoferrate (Ill). The actviation parameters have been evaluated.