With an increasing number of sour oil and gas fields in the world, mitigation of production related failures due to H2S corrosion is a key challenge. In H2S environments, the corrosion product layer could include different types of iron sulfides with various electrical and physiochemical properties. One of the main characteristics of iron sulfides is their semiconductive nature which could enhance the galvanic coupling between steel and this type of corrosion product layer. On that account, galvanic coupling between steel and iron sulfides is considered as the main culprit related to the higher risk of localized corrosion in H2S environments. However, the mechanism of galvanic coupling between steel and iron sulfides are still unclear as the nature of iron sulfides transformation and their electrochemical behavior have not been fully understood yet. The objective of this study is to investigate and model the electrochemical behavior of iron sulfides by specifically focusing on their cathodic characteristics in acidic solutions. Pyrite and pyrrhotite were used as the iron sulfides for these tests since they have been found when localized corrosion of steel was observed in sour pipeline conditions in the field. A rotating disk electrode (RDE) has been utilized for investigation of cathodic reactions occurring on the surface of pyrite, pyrrhotite, and X65 steel. Experiments have been performed in several pH values as well as different rotational speeds in order to characterize the nature of cathodic reactions. In addition, a mathematical model was developed to predict the cathodic current of iron sulfides, and then the results were compared with the experimental data.