Three metal-organic chemical vapor deposition (MOCVD) coatings (ZrO2, Y2O3·ZrO2, Y2O3) were studied in contact with model CMAS to investigate the microstructural evolution and phase formation. The MOCVD coatings were covered with CMAS powder deposits and annealed for 1 h at 1250°C, respectively. The ZrO2 coating was completely infiltrated by CMAS, whereas the yttria containing coatings show a higher resistance against CMAS infiltration. This is explained by the formation of a continuous oxyapatite layer in the reaction zone of the coating and the CMAS deposit. The Y2O3·ZrO2 coating shows the best infiltration resistance despite the fact that the Y2O3–CMAS sample is the only completely crystallized. As crystallization products, oxyapatite, melilite, anorthite as well as new garnets bearing all available cations were formed. The garnet phase was confirmed by XRD and TEM. EDS measurements were used to calculate structural formula A3B2T3O12 (A = Ca, Y, Zr; B = Mg, Al; T = Al, Si).