Martian atmospheric entry of space vehicle requires the study of the oxidation of silicon carbide (main protective material of the thermal heat shield) under CO2 at high temperatures and low pressures. In this paper, experimental measurements of high temperature silicon carbide oxidation under molecular and partially dissociated (by microwave generation) CO2 are realized. The results obtained for the experimental determination of the transition zone between passive and active oxidation under a mixed oxidizing/reducing (CO2/CO) atmosphere are presented. SEM micrographs and XPS analyses are performed on several chosen samples to precisely determine the experimental transition line. Comparison with previous results obtained on the same sintered SiC under microwave dissociated air is achieved to finally find only a little difference (20 K in the studied pressure range) between the behavior under atomic oxygen (dissociated air) and dissociated CO2 for the transition position but the morphology of the surface samples changes significantly when changing the atmosphere nature.