Internally cooled, hollow SiC-based ceramic matrix composites (CMCs) components that may replace metallic components in the hot section of future high-efficiency gas-turbine engines will require multilayered thermal/environmental barrier coatings (T/EBCs) for insulation and protection. In the T/EBC system, the thermally insulating outermost (top coat) ceramic layer must also provide resistance to attack by molten calcia-magnesia-aluminosilicate (CMAS) deposits. The interactions between a potential candidate for top coat made of air-plasma-sprayed (APS) 2ZrO(2)Y(2)O(3) solid-solution (ss) ceramic and two different CMASs (sand and fly ash) are investigated at a relevant high temperature of 1500 degrees C. APS 2ZrO(2)Y(2)O(3)(ss) top coat was found to resist CMAS penetration at 1500 degrees C for 24 hours via reaction products that block CMAS penetration pathways. In situ X-ray diffraction (XRD) studies have identified the main reaction product to be an Ca-Y-Si apatite, and have helped elucidate the proposed mechanism for CMAS attack mitigation. Ex situ electron microscopy and analytical spectroscopy studies have identified the advantageous characteristics of the reaction products in helping the CMAS attack mitigation in the APS 2ZrO(2)Y(2)O(3)(ss) coating at 1500 degrees C. Finally, the Y3+ solubility limit and transport behavior are identified as potential comparative tools for assessing the CMAS resistance ability of top-coat ceramics.