Substitution of Y3+in CaS results in the cubic rock‐salt‐type solid‐solution Ca1−xY2x/3□x/3S, withx0.37 and where □ denotes cation vacancies that form for charge compensation. The substitution range determined from polycrystalline samples prepared at 1025°C corresponds to a solubility limit of 16 mol% Y2S3in CaS. Evolution of the unit cell edge withxindicates that the CaS lattice expands about the cation vacancies. Single‐crystal X‐ray diffraction studies of samples grown from a eutectic CaCl2–KCl flux revealed no indication of superstructure formation, indicating that Ca2+, Y3+, and vacancies are disordered across the cation sites. Thermogravimetric analysis curves for Ca0.7Y0.2S, CaS, and the ternary compound CaY2S4indicate oxidative stability up to 430°, 410°, and