The reaction kinetics and size control in the formation of acicular goethite (α -FeOOH) and lepidcrocite (γ -FeOOH) particle by air oxidation of aqueous suspensions of ferrous hydroxide, were investigated. The formation of α -FeOOH particles may be viewed as a stepwise process involving oxidation of ferrous complex ion, followed by formation of a ferric hydroxo-complex and subsequent hydrolysis of the ferric hydroxo-complex to form goethite. The oxidation rate could be expressed apparently by first-order with respect to the concentration of dissolved oxygen and zero-order with respect to the concentration of total ferrous species. γ -FeOOH is formed via an intermediate green rust. The rates of oxidation steps leading to green rust and γ -FeOOH could also be described apparently as zero-order in the concentration of total ferrous species and first-order in the concentration of dissolved oxygen. The oxidation in the formation of both particles was found to go to completion in the liquid film around the gas bubble, while the formation and deposition of particles was found to mainly take place in the bulk liquid phase. The mean particle size based on the major axis, ranging from 0.15 to 0.30 μm for α -FeOOH and from 0.35 to 0.60 μm for γ -FeOOH, was proportional to the oxidation rate raised to the power -0.30 ~ -0.44. This functional relationship was interpreted on the basis of the concept that the nucleation rate could be expressed as m-order in supersaturation of intermediate species, polynuclear ferric hydroxo-complex. By shifting the oxygen concentration in the feed stream to a lower level at a low conversion, the size distribution of the prepared particles could become sharp. The formation of fine magnetite (Fe₃O₄) particles by similar air oxidation was investigated and discussed from the same points of view.