The crystal structure and thermal expansion properties of solid solutions Sc2?xAlxMo3O12 (x = 0.0-2.0) are systematically investigated by X-ray diffraction. Rietveld refinement results indicate that samples with x ? 1.3 crystallize in a Sc2Mo3O12-type orthorhombic (Pnca) structure, whereas others have an Al2Mo3O12-type monoclinic (P21/a) structure. Substitution of 15 % Al3+ for Sc3+ leads to a rapid change in linear thermal expansion coefficient from ?2.334 ¡Á 10?6 ¡ãC?1 to ?0.732 ¡Á 10?6 ¡ãC?1. The thermal expansion coefficients of Sc2?xAlxMo3O12 monotonously increase with increased Al3+ content.