Due to the serious consequences that calcium-magnesium-alumino-silicate (CMAS) attack has on reducing the service life of thermal barrier coatings (TBCs), it is important to understand the degradation mechanisms of CMAS attack on TBCs. Air plasma sprayed yttria-stabilized zirconia (YSZ) coatings were tested with 20 mg/cm(2) real volcanic ash (VA) deposits at 1150 degrees C and 1250 degrees C for 8 h. The thermo-physical and mechanical properties of the coatings subjected to VA attack were investigated. It was found that VA reacted top coat exhibited apparently higher coefficient of thermal expansion, thermal conductivity, hardness and Young's modulus values than the as-sprayed condition. The microstructure and phase evolution of the coatings were characterized by X-ray diffraction, scanning electron microscope, Raman and transmission electron microscope. The results indicated that Y was leached from YSZ and into the VA melt, which interacted with VA to form yttrium iron garnet, resulting in the detrimental phase transformation from tetragonal to monoclinic.