Fine particles of needle-like lepidocrocite (gamma-FeOOH) were synthesized by the oxidation of aqueous suspensions of ferrous hydroxide using a bubble column with draft tube at a constant temperature ranging from 20 degrees C to 30 degrees C. The oxidation steps leading to green rust tan intermediate) and lepidocrocite (final product), termed Step I and II, respectively, could be described apparently as first order, with respect to oxygen, and zero order, with respect to total ferrous species. When the concentration of oxygen in the feed stream was varied under a constant gas velocity, the mean size based on the major axis of needle-like particle decreased from 0.60 to 0.35 mu m with increasing oxidation rate. When the gas velocity was varied under a constant oxygen concentration, the particle size was almost independent of the oxidation rate and was equal to ca. 0.6 mu m. By the addition of a small amount of sodium dihydrogenphosphate (NaH2PO4), the major axis could be reduced to 0.2 mu m with the minor axis and the oxidation rate almost unchanged.