The synthesis of the Ln(6)MoO(12) (Ln = Sm, Ho -Yb) (3:1) rare-earth molybdates from binary oxides at room temperature has been studied by XRD and electron spin resonance spectroscopy (ESR). The mechanical activation of 3Ln(2)O(3) + MoO3 (Ln = Sm, Ho, Yb) mixtures containing unmilled or premilled MoO3 initiates the formation of Lri(6)MoO(12) (Ln = Sm, Ho-Yb) at room temperature, which is accompanied by a reduction in the ESR signal from Mo5+ paramagnetic ions located on the surface of the activated Mo03 and, hence, by a reduction in the amount of MoO3 in the mixture, due to reaction with Ln(2)O(3) (Ln = Sm, Ho, Yb).The major phase resulting from the mechanochemical synthesis is a cubic phase with the bixbyite structure (la(-3), no. 206) for Ln(6-x)MoO(12-delta) (Ln = Dy -Yb; x = 0, 0.5) and the Sm2O3 type structure (C 2/m, no. 12) for Ln(6)MoO(12) (Ln = Gd, Sm). The high-temperature synthesis of Ln(6-x)MoO(12-delta) (Ln = Ho -Yb; x = 0, 0.5) and Ho1oMo2021 from precursors prepared at room temperature has been studied at 1200 degrees C and heat treatment times of 4, 40, 80, and 160 h. The bixbyite phase disappears because it is metastable and is formed due to kinetic factors in the stability field of lower symmetry phases: tetragonal (T) and rhombohedral (R3). We have located the stability range of the rhombohedral (R3) phase as a function of Ln below 1200 degrees C using different heating time. We have found conditions for the synthesis of the tetragonal phase T, which exists only in the case of the intermediate lanthanides. The tetragonal phase in phase-pure form has been synthesized for the first time via prolonged (160 h) heat treatment of Ho10Mo2O21 at 1200 degrees C. The electrical conductivity of the samples with different % of tetragonal phase has been measured by impedance spectroscopy in dry and wet air. The Arrhenius plot of conductivity for pure tetragonal phase has the form of straight line over the entire temperature range studied, 440-900 degrees C (Ea = 1.37 eV, a 600 degrees C = 1 x 10(5) S cm(-1)) in dry and wet air. The absence of electrode dispersions at low frequencies and conductivity growth in a wet air makes us assume the predominantly electronic conductivity type of the Ho1oMo2021 tetragonal phase in the temperature range 440-900 degrees C. In multiphase samples surface electronic conductivity is observed up to 620-650 degrees C in wet air, which is associated with the presence of defects at grain boundaries of crystallographically related phases (bixbyite, rhombohedral and tetragonal phases).