The major objective of this work was to investigate the initial oxidation behavior of pyrite in order to better understand how mine wastes containing pyrite generate acid water. It was found that a unique potential exists at which a fresh surface of pyrite undergoes neither oxidation nor reduction. This potential is pH dependent and is referred to as the "stable" potential. The stable potential was found to be 0 V at pH 4.6 and -0.3 V at pH 9.2. Fresh, unoxidized pyrite surfaces were obtained by fracturing under the electrolyte while holding at the stable potentials. The initial oxidation behavior of pure pyrite was investigated using cyclic voltammetry technique by beginning potential sweeps from the stable potential and sweeping in the positive-going or negative-going direction. The effects of semiconducting properties of pyrite on its electrochemical behavior were studied by photocurrent measurement. The results indicate that pyrite samples from Peru and Spain are all highly n-type. A spontaneous depletion layer is formed on the fresh surfaces of n-type pyrite. The depletion layer is attributed to an intrinsic, acceptor-like surface state. Charge storage in this surface state pins the band edge over a wide potential range, accounting for the metallic-like electrochemical behavior that has been reported for pyrite. Electrochemical and photocurrent measurements were also conducted on galena for comparison with pyrite. The results indicate that galena samples from the same cluster have varying semiconducting properties, ranging from nearly intrinsic to highly n-type. Abrasion introduces defects in the surface of pyrite and galena, which strongly decrease the magnitude of photocurrent. / Master of Science