The oxidation of ferrous ions in acidic sulfate solutions at elevated air pressures was investigated. The effect of the Fe2+ concentration, Fe3+ concentration 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. A second order of reaction in Fe2+ and a first order of reaction in O-2 were determined, respectively. A slight inhibition by Fe3+ on the Fe2+ oxidation reaction was observed. The reaction is first order in Fe3+ in the kinetic term for the inhibition by Fe3+. Concentrations of H2SO4 up to 1 M result in a fractional negative order of -0.6; concentrations Of H2SO4 above 1 M result in a zero order. One kinetic equation for the oxidation of Fe2+ was postulated, in which the order of reaction in H2SO4 is changed depending on the H2SO4 concentration. R-FE(2+) = -d[Fe2+]/dt = k[Fe2+]P-2(O2)[H2SO4](c)/(1 + A[Fe3+), with c = -0.6 for [H2SO4] <1 M, and c = 0 for [H2SO4] > 1 M. The activation energy was determined to be E-A = 62.1 kJ/mol. The order c in H2SO4 is either -0.6 or zero, depending on the H2SO4 concentration.