The quantitative method of iron in the presence of KCN or CO-gas, as the reserve experiment observing on the isolation of iron in the HbO(2)-KCN-H(2)O(2) reaction system put forward by G. Barkan, was investigated. And the results were as follows. 1. The isolated iron combined with CN and missed the measurement, but the error of measurement could be prevented by the heating process on the fire after the addition of concentrated sulphuric acid and perchloric acid. On the other hand, the measuring error caused by the contain of iron in reagent could be prevented by the use of ammonia instead of caustic soda. 2. It was necessary to stand in the room temperature for 24 hours after the o-phenanthrolin reaction for the measurement of iron by o-phenanthrolin in the reaction solution using phosphate buffer. 3. When the iron dosis in various buffer was measured, the aeration of CO gas had no influence on the measurement. 4. Measuring the dosis of easy split iron by the aeration of CO gas to hemoglobin for 30 minutes, it decreased about one half as compared with that before the aeration of CO gas and it showed only the reversion of 67% by the aeration of oxygen for 60 minutes. On the other hand, it showed the decrease of 70% by the aeration of nitrogen gas, but it completely reverted to the initial dosis by the aeration of oxygen for 10 minutes.