Reactions between molten calcium-magnesium-iron alumino-silicate (CMFAS) deposits and yttrium disilicate (Y2Si2O7, YDS) based environmental barrier coatings (EBC) on SiC/SiC ceramic matrix composites (CMCs) were investigated at 1300 °C. The coating readily dissolves into the melt from which an apatite phase, nominally Ca2Y8(SiO4)6O2, precipitates. These reactions are sufficiently fast to consume the majority of the approximately 275 μm thick coating in 24 h. Liquid phase separation, producing an essentially pure SiO2 second phase, occurs near the reaction front suggesting dissimilar rates of CaO and SiO2 exchange with the overlaying deposit. The rise of large bubbles through the melt above the coatings appears to disrupt the reaction layer and distributes apatite throughout the residual deposit. Channel cracks were found in the deposits and the reaction layers; after longer exposures, the cracks branch and extend laterally through the Si bond coat and into the underlying CMC. Complementary experiments performed on monolithic YDS pellets yielded long-term recession rates similar to those of the coatings, although some differences were evident in recession rates and reaction layer morphologies in the early stages. Thermodynamic calculations were used to understand the evolving driving force for the YDS-to-apatite conversion. The agreement between the simulated and experimentally observed behaviors suggests that such calculations could be used to predict the influence of temperature and deposit composition on EBC degradation.