The impact of nitrate on reservoir souring mitigation and microbially influenced corrosion (MIC) has been investigated in laboratory scale bioreactors. The aim is to link microbial structure and function of biofilms on metal coupons with corrosion damage, metallography, and deposit data. Preliminary results show distinct differences in structure and activity of nitrate-treated relative to control biofilms. Microprofiling studies show strong vertical redox gradients in both nitrate treated and control biofilms, indicating the presence of reducing processes at the metal surface.