Single crystals of FeVMoO7 and CrVMoO7 were grown by a flux method from Fe2O3-V2O5-MoO3 and Cr2O3-V2O5-MoO3 melts, respectively. Single-crystal X-ray diffraction revealed that FeVMoO7 and CrVMoO7 are isostructural, in which V5+ and Mo6+ ions in the [VMoO7](3-) units are crystallographically distinct, and one type of oxygen is bonded only to molybdenum, forming a Mo=O double bond. These rare structural features were also studied by infrared and Raman spectroscopy. Raman Mo=O stretching frequencies are consistent with the assignments made in previous surface studies of molybdate catalysts. Crystal data for FeVMoO7 : triclinic, space group P (1) over bar (No. 2), with a = 5.564(1) Angstrom, b = 6.669(1) Angstrom, c = 7.909(1) Angstrom, alpha = 97.65(1)degrees, beta = 90.33(1)degrees, gamma = 101.32(2)degrees and Z = 2. Crystal data for CrVMoO7 : triclinic, space group P (1) over bar (No. 2), with a = 5.5273(9) Angstrom, b = 6.582(1) Angstrom, c = 7.8650(9) Angstrom, alpha = 96.19(1)degrees, beta = 89.87(1)degrees, gamma = 102.01(1)degrees, and Z = 2.