Sour hydrocarbon reservoirs, containing H2S gas, are receiving increasing attention due to the growing demand for energy. Although uniform corrosion is not a significant obstacle for oil and gas companies in sour environments, the major challenge in this field is prevention of localized corrosion that can cause failures in production infrastructure. Formation of different types of iron sulfides as corrosion products has been postulated to be a main culprit for localized attack, due to their wide-ranging physicochemical properties, such as their electrical conductivities. The galvanic coupling between iron sulfides layers and mild steel was shown to be the mechanism associated with the localized attacks in sour environments. However, the mechanism of galvanic coupling between mild steel and iron sulfides has not been understood due to the lack of systematic and parametric studies. The research described herein addresses the galvanic coupling between mild steel and iron sulfide polymorphs to investigate the involved mechanisms. For the first step, the uniform corrosion of mild steel in aqueous H2S solutions was critically reviewed, and the mechanism associated with this system including the involved chemical electrochemical reactions was fully described. Furthermore, the mathematical modeling of uniform corrosion rate of mild steel in H2S environments was also comprehensively reviewed. In addition, some new approaches for the modeling of electrochemical reactions as well as corrosion rate were proposed, and the validity of the proposed models were verified through comparing with experimental data. Next step of the current research focused on the galvanic corrosion between mild steel and iron sulfides using both experimental and modeling investigations. In the experimental section, a systematic study was performed in order to determine the contribution of the influential parameters on the galvanic corrosion between mild steel and iron sulfides. On that account, the effect of several important parameters including iron sulfide type, cathode to anode surface area ratio, and solution’s conductivity was examined. Although the conductivity of solution did not show significant impact on the galvanic corrosion rate in the studied experimental conditions, the type of iron sulfide as well as the cathode to anode surface area ratio notably affected the galvanic coupling process. Specifically, the two types of iron sulfides used here, pyrite and pyrrhotite, were shown to produce different galvanic corrosion due to their different electrochemical behaviors. To further elucidate the mechanism of the impact of iron sulfide type, the cathodic behavior of pyrite and pyrrhotite was systematically studied using rotating disk electrode system. The results revealed that pyrrhotite has significantly higher cathodic current than pyrite. This difference originates from different cathodic reactions occurring at the surface of these iron sulfides. Therefore, the associated cathodic reactions were also proposed in this study. The modeling section started with the proposed model for the cathodic current density of mild steel, pyrite, and pyrrhotite. The mathematical model was based on the proposed cathodic reactions as well as the constants found from experimental investigation. The proposed model was then verified by comparing it to the experimental data obtained in this study. In the final part of the current study, a mathematical model based on the polarization measurements was proposed for the prediction of galvanic corrosion rate between mild steel and iron sulfides. The model was compared with the experimental results at various experimental conditions, and very good agreement was found between experimental and modeling results. To summarize, the current research was ale to reveal some mechanistic aspects of the galvanic coupling between mild steel and iron sulfides. The impact of the influential parameters and the electrochemical characteristic of iron sulfides were revealed. Also, the current research, for the first time, developed a mathematical model for the prediction of cathodic current on iron sulfides and the galvanic corrosion rate between mild steel and iron sulfides.