A solid solution was found to exist in the quaternary Li2O-MgO-V2O5-MoO3 system between the two phases Mg2.5VMoO8 and Li2Mg2(MoO4)(3). Both Mg2.5VMoO8 and Li2Mg2(MoO4)(3) are isostructural with the mineral lyonsite, and substitution according to the formula rectangle(1/4-x/6)Li(4x/3)Mg(15/4-7x/6)V(3/2-x)Mo(3/2+x)O(12) (0 <= x <= 1.5, where rectangle denotes a cation vacancy) demonstrates that a complete solid solution exits coupling the addition of molybdenum and lithium with the subtraction of cation vacancies, magnesium, and vanadium and vice versa. Vibrational Raman spectroscopy indicates that molybdenum-oxo double bonds preferentially associate with the cation vacancies.