To investigate the influences of metal ions on beta-FeOOH formation, the beta-FeOOH particles were prepared by aerial oxidation of FeCl2 solutions in the presence of chlorides and sulfates of Ti(IV), Fe(III), Cu(II), and Ni(II), and CrCl3 at different metal/Fe atomic ratios of 0-0.1 and characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, inductively coupled plasma, thermogravimetry and differential thermal analysis, and N-2 adsorption. The produced particles contained a large quantity of Ti(IV) and Cr(III) but little Cu(II) and Ni(II). All the added metal chlorides and sulfates made the formed particles less crystalline. The sulfate and chloride of Ti(IV) inhibited the formation of beta-FeOOH at a high Ti/Fe by a marked depression of solution pH by hydrolysis of Ti(IV). The metal sulfate ions effectively reduced the crystallinty of beta-FeOOH to give highly microporous particles. The strong inhibitory effect of SO42- on the crystallization of beta-FeOOH can be ascribed to the adsorption of SO42- on the primary particles. This is also evident from the high content of SO42- in the formed particles. (C) 2004 The Electrochemical Society.