When sulfide minerals are exposed to the oxidizing conditions of the earth's surface, their metal ions are released into solution and the S²⁻ is oxidized to either elemental sulfur or sulfate. The experiments described here used a mixed flow reactor system to determine the oxidation rates of galena and sphalerite under conditions similar to that expected in a weathering ore deposit . The specific surface area of the run solids was determined by N₂ BET procedure and the surface textures observed by SEM. The amount of Fe³⁺ converted to Fe²⁺ by the oxidation reaction was determined using an Eh electrode. Solid reaction products include, orthorhombic S(s) and anglesite (PbSO₄) from the galena oxidation and minor orthorhombic S(s) from the sphalerite oxidation. The rate equations describing the 25°C data are: dn_Fe³⁺/dt = -5.5 ± 1.1 × 10⁻³ (A)(a<sub>Fe³⁺</sub)^{1.06 ± 0.16} for galena, and dn_Fe³⁺/dt = -1.8 ± 0.3 × 10⁻⁶ (A)(a_Fe³⁺)^{0.47 ± 0.08} for sphalerite. Where dn_Fe³⁺/dt is the rate of reduction of Fe³⁺ (moles sec⁻¹), and A is the surface area of the solid (m²). The calculated E_a for galena oxidation is 48 kJ mol⁻¹ (25 - 40°C) and is 84 kJ mol⁻¹ (25 -60°C) for sphalerite oxidation. Although galena and sphalerite are both simple, cubic, monosulfides their reaction rate with ferric iron differs by about 2.5 orders of magnitude for m_Fe³⁺ = 10⁻³. / M.S.