Abstract: Potentio-titrimetry was applied to the determination of hydroxyl free radical.Excess of 1.0×10-4mol·L-1 ferrous sulfate solution was added to react with the hydroxyl free radical (·OH) and the un-reacted amount of ferrous sulfate was titrated potentiometrically with 1.0×10-4mol·L-1 ceric sulfate solution in the solution acidified with sulfuric acid.It was found that the optimum acidity of the solution for the redox reaction between ·OH and Fe2+ was pH 4,and that the time taken for completion of the reaction was 8 min.Based on these facts,acetate buffer was used and the back-titration with ceric sulfate solution was performed 10 min after the starting of reaction.In the determination of ·OH by the proposed method,value of RSD (n=10) found was 0.26%.Detection limit (3s/b) of the method was found to be 3.1×10-7mol·L-1.