When air was bubbled into a ferrous ion solution maintained at pH = 10.0, magnetite was precipitated due to oxidation of ferrous ion in the aqueous phase. The total concentrations of ferrous and ferric ions were measured over time in a three-phase system (air, solution and precipitated magnetite). The total concentration of ferrous and ferric ions linearly decreased with time until about 10% of the initial ferrous concentration remained according to the equation, [Fe] = [Fe](0)(1 - alpha t) The overall rate constant, a, increased with a decrease in the initial ferrous concentration and with an increase in the flow rate of air. The overall rate constant could be interpreted by the reaction model taking into account the oxygen absorption rate as well as chemical reaction rates in the aqueous phase. The overall reaction rate was limited by the oxidation rate of ferrous ion. The rate constant of the oxidation depended on the initial ferrous concentration, the air flow rate and the air dispersion state. The decrease in the magnetite formation rate in the late stage may be interpreted qualitatively as being due to the surface of the ferrous hydroxide becoming covered with magnetite.