The redox potential and pH of aerated suspensions of iron(II) hydroxide in sulphate-containing aqueous solutions are measured during the oxidation process. Plateaux corresponding to the equilibrium conditions between Fe(OH) (sub 2(s)) and Fe(II)-Fe(III) hydroxysulphate GR2(SO (super 2-) 4 ) (sub (s)) on the one hand, and between GR2(SO (super 2-) 4 ) (sub (s)) and FeOOH (sub (s)) on the other hand, are displayed. Potentiometry, voltammetry, pH-metry and Mossbauer spectroscopy are applied to follow all reactions. The thermodynamic meaning of the measured potential of the first plateau which corresponds to the GR2(SO (super 2-) 4 ) (sub (s)) /Fe(OH) (sub 2(s)) equilibrium is demonstrated. The chemical composition of GR2(SO (super 2-) 4 ) (sub (s)) is found to be Fe 4II Fe 2III (OH) 12 SO 4 .nH 2 O all along the oxidation process, implying that this compound must be considered as a pure phase with a well-defined composition. The Gibbs standard free energy of formation or chemical potential mu degrees [GR2(SO (super 2-) 4 ) (sub (s)) ] in "anhydrous form" (n = 0) is determined at -3790+ or -10 kJ mol (super -1) . A consistent value of mu degrees [Fe(OH) (sub 2(s)) ] at -490+ or -1 kJ mol (super -1) is obtained.