Abstract Oil and gas production facilities transport a wide range of fluids which may contain carbon dioxide (CO2), hydrogen sulfide (H2S) and many other species that may lead to a corrosive environment. To ensure safe and cost effective production operations it is important to understand the combined effect of H2S and CO2 on corrosion rates and the corrosion products formed in these environments. The formation of iron sulfide (FeS) and iron carbonate (FeCO3) scale under a corrosive environment is one of the important factors governing corrosion rates in CO2 and H2S corrosive systems. This paper presents a simplified thermodynamic model to predict the dominant mineral on the corroding surface exposed to a range of mixed partial pressures CO2 and H2S. The approach is based on thermodynamic estimate of equilibrium constants i.e. Ksp for FeCO3 and Ksp for FeS to account for the formation of iron sulfide and iron carbonate scales at varying temperature, pH, iron concentrations and partial pressure of CO2 and H2S. The application and limitations of this model as a methodology to predict the dominant scale (i.e. iron sulfide or iron carbonate) on the corroding surface is discussed.