Phenylphosphonous acid and chromium(VI) react in aqueous perchloric acid as follows: 3 PIII+ 2 CrVI→ 3 PV+ 2 CrIII. The oxidation is effected by the HCrO4– ion. Complex formation between phenylphosphonous acid and chromium(VI) is suggested and the empirical rate law is as shown (i). The rate is proportional to the –d[CrVI]/dt=k3K[CrVI][PhPO2H2]/(1 +K[PhPO2H2])(i) first power of hydrogen-ion concentration. The effect of solvent composition on the reaction rate indicates the reaction to be of ion–dipole type. Chloride, sulphate, and phosphate ions decreased the rate considerably whereas an incrase in rate in the presence of perchlorate ion has been observed. The activation parameters have been evaluated.