The relative reactivities of pulverized samples (100-200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH = 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10⁻³ molal. The rate of the reaction: FeS₂ + 14Fe³⁺ + 8H₂O = 15Fe²⁺ + 2SO₄²⁻ + 16H⁺ was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law: -d m_Fe³⁺ / dt = k (A/M) m_Fe³⁺ where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and A/M is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0x10⁻⁴ to 2.7x10⁻⁴ sec⁻¹ ±5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different. The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol⁻¹. The BET-measured specific surface area for lower-temperature/ early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater A/M ratio, they will appear to be more reactive per unit mass than the higher temperature types. / Master of Science