To establish an adequate thermodynamic model for the mechanism of formation of hydrogen sulfide (H2S) corrosion products, theoretical and experimental studies were combined in this work. The corrosion products of API X60 pipeline steel formed under different H2S corrosion conditions were analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. A thermodynamic model was developed to clarify the environmental boundaries for the formation and transformation of different products. Presumably, a dividing line with a negative slope existed between mackinawite and pyrrhotite. Using experimental data presented in this study combined with previously published results, we validated the model to predict the formation of mackinawite and pyrrhotite on the basis of the laws of thermodynamics. The established relationship is expected to support the investigation of the H2S corrosion mechanism in the oil and gas industry.