Synthesis of single crystals in the lithium-rich part of the system LiVO₃-NaVO₃ and determination and refinement of the crystal structures for four compositions with x = 0.15, 0.66, 1.40 and 2.00 (space group C2/c, diopside structure type) show that there is unlimited (perfect) isomorphic substitution which is rare for Li-Na. The substitution is ordered throughout the series: Li atoms are situated primarily in the more regular M1 octahedra and the Na atoms in the M2 octahedra.-A.P.