A series of compounds of Co1-xMgxMoO4 compositions has been prepared by a conventional ceramic route. The members of the whole solid solution exhibit a reversible first-order phase transition which was probed by using thermal expansion and low-temperature reflectivity techniques. Whereas the alpha -> beta transition temperature evolves linearly on warming from 435 to 200 degrees C with x going from 0 to 0.9, the beta -> alpha transition temperature variation falls,down on cooling from -40 degrees C to -140 degrees C going from CoMoO4 to Co0.1Mg0.9MoO4 with an asymptotic evolution. The phase transition temperatures have been explained on the basis of a crystal polarization effect under substitution of Mg for Co. Thus, from an applicative point of view, new thermochromic pigments with tunable transition temperatures are here proposed.