The interaction of molten calcium-magnesium alumino-silicate (CMAS) with yttrium monosilicate (YMS), a candidate environmental barrier coating (EBC) for SiC-based ceramic matrix composites, was investigated. YMS monoliths were exposed to CMAS melts at 1300 degrees C for times ranging from 1 to 100 h and characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy/electron microprobe analysis, and electron back-scattered diffraction. YMS is dissolved into CMAS and reprecipitates as a Ca(2)Y(8)(SiO(4))(6)O(2) oxyapatite phase. A nominally continuous layer of apatite is readily established at the interface, but its effectiveness as a diffusion barrier is compromised by the presence of thin amorphous films along the grain boundaries that enhance chemical transport between the bulk melt and the YMS. Moreover, the growth mechanism involves continuous renucleation that eventually leads to separation of the crystals formed previously from the interface and their subsequent coarsening. The latter results in widening of the amorphous boundary films, enhancing interdiffusion. The significance of these findings on the viability of YMS as a durable EBC is discussed.