The SiMo cast iron is a spheroidal graphite cast iron which major alloying elements are silicon and molybdenum. This alloy is currently used in diesel engines as part of exhaust gas system like exhaust manifold or turbocharger housing components. It is then designed to work at high temperature under corrosive atmosphere. However, this alloy presents sometimes unusual high corrosion rates, and it is thus advisable to define the critical conditions leading to this type of problem. In the present study, the SiMo cast iron is tested in complex atmospheres containing N2, O2, SO2, CO2 and H2O in a temperature range lying between 300°C and 600°C. SEM observations and EDX and XRD analyses allow to qualify the nature of the corrosion products. Some thermodynamic calculations complete the experimental results. Three experimental parameters are especially studied in this work: the temperature of the isothermal corrosion tests, the presence of water vapour and the SO2 content in the gas mixture. The main results show that the corrosion products are metallic sulphides, sulfates and oxides. When sulphides and / or sulfates are present, the corrosion scale does not ensure a sufficient protection of the substrate and it is necessary to find conditions where only oxides are present. Such conditions are obtained for the highest temperature used in this study (600°C), or with highly oxidant atmospheres.