The reactivity between rare-earth (RE-) oxide stabilized ZrO2 or HfO2 thermal barrier coatings (TBCs) and a calcium-magnesium-aluminum-silicate (CMAS) melt was studied at 1310 degrees C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200 degrees C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE=Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top-coating system. Thermodynamic calculations of the phase assemblage of the (1-x) ZrO2-xY(2)O(3) coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE-oxide content higher than 16mol% in the reaction zone for the ZrO2 system. In general, samples with nominal compositions ZrO2-9Dy(2)O(3), HfO2-7Dy(2)O(3), ZrO2-8Y(2)O(3), HfO2-6Er(2)O(3), ZrO2-9.5Y(2)O(3)-2.25Gd(2)O(3)-2.25Yb(2)O(3), and ZrO2-30Y(2)O(3) exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.