The oxidation of ferrous ions in acidic sulfate solutions at elevated air pressures was investigated. The effect of the Fe2+ concentration, initial H2SO4 concentration and partial oxygen pressure on the reaction rate were determined at three different temperatures, that is, T = 90, 70, and 50 degrees C. The effect on the reaction rate of the components that H2SO4 dissociates into, that is, HSO4-, H3O+, and SO42-, was established as well. A second order of reaction in Fe2+ and a first order of reaction in O-2 were determined. No clear order in either H2SO4 or the components H2SO4 dissociates into, could be established. For the.experiments with initial concentrations of H2SO4 of 1 M and higher the oxidation rate was not affected, that is, a zero order of reaction in H2SO4 for these concentrations. Therefore, the kinetic rate expression for the oxidation of Fe2+ at concentrations of H2SO4 of 1 M and higher can be calculated with R-Fe(2+) = d[Fe2+]/dt = k[Fe2+]P-2(O2), where the activation energy E-A was determined to be 60.3 kJ/mol.