Amorphous, α, β, and γ-phase yttrium disilicate Y2Si2O7 pellets were synthesized by spark plasma sintering, and their chemical durability and degradation mechanisms were investigated via semi-dynamic leaching test in pH = 3 nitric acid at 90 °C. All crystalline phases displayed relatively congruent dissolution with a slight preferential release of Y. The amorphous sample showed clear incongruent dissolution with preferential release of Y, leading to a surface reorganization reaction and the formation of a SiO2 passivation layer 20-25 μm thick on the surface of the sample. This passivation layer led to a continuous decrease in the elemental release rates of the amorphous sample. The γ-phase sample displayed the lowest short-term and long-term leaching rates of Y and Si, followed by the β, α, and then amorphous sample. The crystalline samples showed increasing release rates over time indicating a dissolution-controlled reaction mechanism. Post-leaching microstructural characterization of the β and γ samples revealed variations in the corrosion levels of the surface grains, indicating a dependence on grain orientation.