The rates of oxidation of arsenopyrite and chalcopyrite by Fe³⁺ at concentrations of 10⁻² to 10⁻⁵ molal with a pH near 2 and temperatures of 0 to 60°C, conditions similar to those found in weathering sulfide ore deposits, have been determined. The rate equations for 25°C are: dn_Fe3+/dt = 1.9±0.4 x 10⁻⁷ (A) (m_Fe3+)_0.43±0.01 dn_Fe3+/dt = 1.5±0.4 x 10⁻³ (A) (m_Fe3+)_0.94±0.04 for chalcopyrite and arsenopyrite respectively. The E_a of chalcopyrite is 63±2 kJ mol⁻¹ (40 - 60°C). The E_a of arsenopyrite varied with temperature from 18±2 kJ mol⁻¹ (0 - 25°C) to -6±2 kJ mol⁻¹(25 - 60°C). / M.S.