Xenotime rare-earth (RE) phosphates are emerging as promising materials for environmental barrier coatings (EBCs) for SiC-based ceramic–matrix composites (CMCs) because of their close coefficients of thermal expansion (CTEs) and resistance to calcium–magnesium–alumina-silicate (CMAS) corrosion. In this work, high-entropy (HE) (Sc_0.2Lu_0.2Yb_0.2Er_0.2Y_0.2)PO₄ and five single-component REPO₄ (RE = Sc, Lu, Yb, Er, and Y) compounds were synthesized, and their stability, thermal properties, and CMAS corrosion resistance were investigated. The CTE values of four REPO₄ compounds (RE = Lu, Yb, Er, and Y; ~(5.6–6)×10⁻⁶ °C⁻¹) are close to those of SiC–CMC ((4.5–5.5)×10⁻⁶ °C⁻¹), whereas ScPO₄ (6.98×10⁻⁶ °C⁻¹) and HE (5RE_0.2)PO₄ (6.39×10⁻⁶ °C⁻¹) have slightly higher values in the temperature range of 200–1300 °C. HE phosphate has the lowest thermal conductivity due to its size and mass disorder. Systematic CMAS corrosion tests at 1300 °C for 5, 45, and 96 h revealed that all RE phosphates formed a continuous and dense reaction layer predominantly composed of Ca₈MgRE(PO₄)₇, effectively impeding CMAS penetration. Additionally, REPO₄ with smaller RE³⁻ cations displays a slower reaction rate and reduced corrosion kinetics, as evidenced by the smaller thickness of the reaction layer. A larger negative difference in the optical basicity (OB) value between REPO₄ and CMAS signifies greater corrosion resistance. A mechanistic understanding of CMAS corrosion and elucidation of the effects of critical parameters such as the ionic mass and ionic radius of RE elements on their thermal properties and CMAS corrosion kinetics are useful for the development of novel xenotime-type phosphates as EBCs for SiC–CMCs.