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Abstract Phase stability, thermal properties, and calcium–magnesium–alumina–silicate (CMAS) resistance of LuPO₄at 1300°C, 1400°C, and 1500°C were studied to evaluate its potential as an environmental barrier coating (EBC) for SiC‐based ceramic‐matrix composites (CMCs). Its coefficient of thermal expansion (∼5.69 × 10⁻⁶°C⁻¹) is close to that of SiC‐based CMCs. At 1300°C, a dense reaction layer of Ca₈MgLu(PO₄)₇forms and inhibits CMAS penetration; however, no such layer forms at 1400°C and 1500°C, leading to CMAS infiltration along grain boundaries. Prolonged (45 and 96 hours) CMAS corrosion of LuPO₄at 1300°C showed the formation of a disilicate (Lu₂Si₂O₇) phase along with Ca₈MgLu(PO₄)₇. A multicomponent rare earth phosphate (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄shows improved CMAS resistance at 1400°C due to higher grain boundary stability and slower dissolution rate of rare earth elements into molten CMAS than single component rare earth phosphate. The mechanisms of CMAS corrosion and the kinetics of the formation of protective reaction layers in LuPO₄and (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄were elucidated. Multicomponent design is needed to increase grain boundary stability and reduce dissolution rate into molten CMAS for REPO₄‐based EBCs.