Lanthanum molybdate, La2Mo2O9, has been attracted considerable attention owing to its high concentration of intrinsic oxygen vacancies, which could be reflected by enhanced phonon scattering and low thermal conductivity. A new series of La2 Mo2O9-based oxides of the general formula La2-xSmxMo2-xWxO9, where x <= 0.2, were synthesized by citric acid sol-gel process. The variation in thermal conductivity with Sm3+ and W6+ fractions was analyzed based on structure information provided by X-ray diffraction and Raman spectroscopy. The fully dense La2-xSmxMo2-xWxO9 ceramics showed a minimum thermal conductivity value [kappa = 0.84 W . (m.K)(-1), T = 1073 K] at the composition of La1.8Sm0.2Mo1.8W0.2O9, which stems from the multiple enhanced phonon scatterings due to mass and strain fluctuations at the La3+ and Mo6+ sites as well as the high concentration of intrinsic oxygen vacancies embedded in the crystal lattice. The thermal conductivities present an abrupt decrease at the structural transition, which is due to the phase transformation from a low-temperature ordered form (monoclinic alpha-La2Mo2O9) to a high-temperature disordered form (cubic beta-La2Mo2O9).