Reactions between silicate deposits and environmental barrier coatings (EBCs) for SiC/SiC composites can severely degrade coating performance. The goal of the present study is to elucidate effects of deposit composition and exposure temperature and time on the recession of yttrium disilicate (YDS), a candidate EBC material. Phase equilibrium calculations are used to predict reactions of YDS with deposits of twelve different compositions at 1300 °C and 1400 °C; experimental observations are reported for three exemplary compositions at both temperatures and compared with the thermodynamic predictions. For initial deposit thicknesses of about 100 μm, recession depths reach terminal values after exposures of about 100 h. Terminal recession depths are sensitive to deposit composition but are only weakly affected by temperature. Deposits with high initial Ca:Si ratios react most severely with YDS, forming thick layers containing apatite interpenetrated by residual melt, whereas reactions with deposits that have Ca:Si ratios below a threshold for apatite formation are more benign, albeit still significant.