This work presents theoretical and experimental results on the speciation of the Fe(II)-Fe(III)-H2SO4-H2O system in concentrated solutions (up to 2.2 m H2SO4and 1.3 m Fe). The aim was to study the chemical equilibria of iron at 25 and 50 °C in synthetic aqueous sulphuric acid solutions that contain dissolved ferric and ferrous ion species. Raman spectroscopy, volumetric titration and conductivity measurements have been carried out in order to study the presence of specific ions and to characterize the ionic equilibrium. A thermochemical equilibrium model incorporating an extended Debye-Hückel relationship was used to calculate the activities of ionic species in solution. Model calculations were compared with experimental results. Model simulations indicate that anions, cations and neutral complexes coexisted in the studied system, where the dominant species were HSO4-, H+, Fe2+and FeH(SO4)20. This indicated that these solutions showed a high buffer capacity due to the existence of bisulphate ions (HSO4-), which presented the highest concentration. A decrease in the concentration of H+and Fe3+took place with increasing temperature due to the formation of complex species. Standard equilibrium constants for the formation of FeH(SO4)20were obtained in this work: log Kf0= 8.1 ± 0.3 at 25 °C and 10.0 ± 0.3 at 50 °C. © 2005 Elsevier B.V. All rights reserved.