Yttrium silicates are promising candidate materials for oxidation and erosion protection of carbon fiber reinforced composites. Such composites are used for the construction of hot structures for space vehicles. Low pressure plasma sprayed yttrium silicate on CVD-SiC pre-coated C/C-SiC materials was investigated by high temperature experiments and thermodynamic calculations. A dataset for the Y-Si-C-O system including the silicates Y2SiO5 and Y2Si2O7 was established and the phase stability ranges and chemical compatibilities of the coatings and the substrate material were calculated. The influence of the oxygen partial pressure on possible interface reactions was taken into account. The experiments show that the coating system is protective under atmospheric re-entry conditions and well suitable up to temperatures of 1650°C. At higher temperatures, however, the experiments show blister formation on the surface of the materials and severe coating spallations. The materials behavior can be explained by interface reactions and the accompanied gas phase formation.