Study by X-ray diffraction of various mixtures of CaS and Yb2S3 after heating at 1200 degrees C, reveals the existence of continuous domains of solid solutions with successively a NaCl-type cubic unit cell (parameter a), a cubic unit cell (parameter 2a) and a rhombohedral structure; Some of the structures have been determined by three dimensional X-ray diffraction. Ca3.86Yb0.09S4 [a = 5.691 Angstrom, Fm3m] and Ca3.10Yb0.60S4 [a = 5.675 Angstrom, Fm3m] have a rock salt structure. In Ca2.30Yb1.14S4 [a = 11.246 Angstrom, ], Ca and Yb are in the same octahedral (24f) sites; octahedral (4c) and (4d) positions are occupied respectively by Ca and Yb atoms. The order of the cations in the [111] planes accounts for the superstructure.