Sulphuric water pickling liquor containing ferrous sulphates was oxidized by Thiobacillus ferrooxidans at different pHs. The oxidized solution was then evaporated and crystallized, and products characterized by X-ray powder diffraction, X-ray high temperature powder diffraction and Fourier transform infrared (FTIR) spectrometry. The thermal decomposition of these materials, studied by calorimetric and thermogravimetric techniques, shows that iron(III) hydroxysalts and monoammonium and triammonium salts are formed during the oxidation-evaporation-crystallization process. However, this formation depends upon pH.