The transformation of gamma-FeOOH into alpha-FeOOH in FeSO4 solutions at 50degreesC was investigated in two different ways by dissolving Ti(IV), Cr(III), Cu(II), Ni(II), and Mn(II) in the solutions at atomic ratios of metal/Fe of 0-0.1 and adding these metal ions to the gamma-FeOOH particles. The transition into alpha-FeOOH was examined by Fourier transform infrared spectroscopy and X-ray diffraction. Ti(IV), Cr(III), and Cu(II) dissolved in the solutions markedly interfered with the transition into alpha-FeOOH, whereas Ni(II) and Mn(II) showed no influence. The inhibitory effect of the former three metal ions is ascribed to the protection of gamma-FeOOH particles against dissolution in FeSO4 solution by surface coating with the hydrolysis species of metal ions, because transmission electron microscopy and inductively coupled plasma-Auger electron spectroscopy demonstrated that the conversion proceeds by dissolution and recrystallization. The metal ions added in gamma-FeOOH particles promoted the conversion into alpha-FeOOH, which was interpreted by the enhancement of dissolution of gamma-FeOOH particles due to the decrease in particle size by the addition of metal ions. (C) 2004 The Electrochemical Society.