Silicon and molybdenum (SiMo) alloyed cast irons with spherical graphite are used for exhaust manifolds for service at high temperatures in air and combustion atmospheres containing water vapor. Analysis of degraded surfaces of in-service manifolds indicates existence of a combined oxidation and de-carburization (de-C) phenomena. Therefore, sequential high-temperature tests in air and combustion atmosphere with recording weight change together with carbon analysis at each time step were utilized to quantify the kinetics of both processes. The recorded weight change was related to weight gain due to oxidation and weight loss due to de-C. Carbon analysis was used to de-couple these two processes. SEM measured thicknesses of de-C layers were used to verify kinetics obtained from changes in carbon concentration during oxidation. It was shown that the oxidation and de-C kinetics have different sensitivities to testing temperature and the type of oxidizing atmospheres. At 700 °C and above, there are several significant mutual effects between scale formation and de-C processes which affect the kinetics of these processes and activation energy. The tested experimental methodology for decoupling these processes can be used for alloy optimization.