Investigation of the First-Order Phase Transition in the Co 1-x Mg x MoO 4 Solid Solution and Discussion of the Associated Thermochromic Behavior. -Pure β-Co1-xMgxMoO4 (0 ≤ x ≤ 1) solid solutions are prepared by solid state reactions of stoichiometric mixtures of MoO3, Co3O4, and MgO (alumina crucible, 700 °C, 20 h). The α-form of the solid solutions is obtained at room temperature from the β-form after either manual grinding or quenching in liquid nitrogen. The samples are characterized by powder XRD. Both forms crystallize in the space group C2/m. The β-CoMoO4 phase is violet and the α-CoMoO4 phase is dark green. Mg substitution is used to tailor the color change in this family of compounds. β → α and α → β transitions are probed by reflectivity on cooling and dilatometry on warming, respectively. The origin of the color change as well as the evolution of the transition temperature as a function of x is discussed in terms of the ionocovalency of M-O (M: Mg, Co) and the Mo-O bonds, and order-disorder transition. The title solid solutions may be new thermochromic pigments with tunable transition temperature. -(