Sr₃MnMo₂O₉ and Ba₃MnMo₂O₉ double perovskites with Mo⁵⁺ valence states have been prepared in polycrystalline form by thermal treatment, in reducing atmosphere (H₂/N₂ flow), of previously decomposed citrate precursors. These materials have been studied by X-ray and neutron powder diffraction (XRPD, NPD), magnetic and transport measurements. At room temperature, the crystal structure of Sr₃MnMo₂O₉ is monoclinic, space group P2₁/n, with a = 5.6564(1) Å, b = 5.6383(1) Å, c = 7.9765(2) Å, β = 89.994(7)º; whereas Ba₃MnMo₂O₉ presents a cubic lattice, space group Fm3¯m, with a = 8.14900(8) Å. These samples present a frustrated magnetic interaction below to 12 and 9 K for Sr₃MnMo₂O₉ and Ba₃MnMo₂O₉, respectively. Semiconductor behaviour, observed either from d.c. or a.c. measurements, was modelled by a variable-range hopping mechanism. This transport behaviour is in accord with the electronic configurations Mn²⁺(3 d⁵)–Mo⁵⁺(4 d¹) and supports the absence of mixed valence states in both double perovkites.