Rare-earth oxides are intrinsically hydrophobic, and this characteristic opens new horizons for their design and diverse applications as robust hydrophobic surfaces. Here, we discover some rare-earth (RE) monosilicates (RE2SiO5) that are hydrophobic under ambient conditions. Their hydrophobicity is positively correlated with their hot-steam corrosion at 1400 degrees C. RE3+ in RE2SiO5 is further found to be a hydration prohibitor, and its species can regulate hydration and volatilization at very high temperatures. These results may provide basis for innovative designs of RE silicates in harsh hot-steam environments for robust environmental barrier coatings (EBC) or as ceramic components.