Acicular goethite particles were obtained by oxidation of weakly alkaline solutions containing varying amount of ferrous hydroxide. The change in pH during oxidation was analyzed in detail with special emphasis on the behavior of green rust (II), the metastable intermediate. Green rust (II) was stabilized by the buffering function of Fe2+ in the solution. Coexistence of green rust (II) with goethite enables steady growth of goethite preferentially in the axial direction, resulting in an increase in the aspect ratio.