Abstract: Rare-earth silicates are promising environmental barrier coatings (EBCs) that can protect SiC_f/SiC_m substrates in next-generation gas turbine blades. Notably, RE₂Si₂O₇ (RE = Yb and Ho) shows potential as an EBC due to its coefficient of thermal expansion (CTE) compatible with substrates and high resistance to water vapor corrosion. The target operating temperature for next-generation turbine blades is 1400°C. Corrosion is inevitable during adhesion to molten volcanic ash, and thus, understanding the corrosion behavior of the material is crucial to its reliability. This study investigates the high-temperature corrosion behavior of sintered RE₂Si₂O₇ (RE = Yb and Ho). Samples were prepared using a solid-state reaction and hot-press method. They were then exposed to volcanic ash at 1400°C for 2, 24, and 48 h. After 48 h of exposure, volcanic ash did not react with Yb₂Si₂O₇ but penetrated its interior, causing damage. Meanwhile, Ho₂Si₂O₇ was partially dissolved in the molten volcanic ash, forming a reaction zone that prevented volcanic ash melts from penetrating the interior. With increasing heat treatment time, the reaction zone expanded, and the thickness of the acicular apatite grains increased. The Ca:Si ratios in the residual volcanic ash were mostly unchanged for Yb₂Si₂O₇ but decreased considerably over time for Ho₂Si₂O₇. The Ca in volcanic ash was consumed and formed apatite, indicating that RE³⁺ ions with large ionic radii (Ho > Yb) easily precipitated apatite from the volcanic ash.