SiC(m)/SiC(f) ceramics matrix composites (CMC) are intended to be used more and more frequently to make parts located in hot zones of the next generations of reactors. However, in combustion environment, rich in water vapor, these materials are degraded by oxidation and volatilization phenomena. In order to increase the lifetime of these composites, protective coating acting as an environmental barrier (EBC) is generally deposited by plasma spraying technics or liquid coating processes, on the surface of a CMC/bond coat (silicon) architecture. In regard to the extreme environmental conditions in an aircraft engine (mechanical loading and corrosion degradation), the development of EBCs is currently focused on materials based on rare earth silicate as yttrium or ytterbium silicates (RE2Si2O7 and RE2SiO5 with RE: Rare Earth). They are thermochemical and thermomechanical compatible with the whole architecture. The objective of this work is to identify the corrosion mechanisms of new systems by molten sands (calcium magnesium alumino silicate : CMAS). To get an efficient protection, the coating has to be fully dense. Thus, alumina has been introduced in different quantities through the use of various compounds to get a reactive sintering and/or transient liquid phase during this densification process. To understand the behavior, at high temperature in contact with molten sands, of developed systems, the corrosion resistance of each constituent introduced in EBC and of new EBC compositions was characterized. Corrosion tests under air were carried out on samples coated with 20mg.cm2 of CMAS, with various compositions, at 1400°C up to 100h. The chemical interactions between the different EBC materials and CMAS were characterized by measuring the recession rates of EBC, the infiltration depths of the liquid oxide inside the EBC and by identifying the dissolved phases and the new ones precipitating. Thermodynamic calculations on the solid-liquid equilibria confirmed the release of certain phases and the formation of others. The variations of molar volumes between these different involved phases allowed to discuss the capability of these new formed phases  to generate on homogeneous coating over the EBC surface, able to limit the further propagation of the corrosion front. Overall, few relations could be established between EBC composition/dissolution/precipitation /protection in function of the CMAS composition.