Composites with the general formulas (1 – x)La2(WO4)3–xSiO2 and (1 – x)Sm2(WO4)3–xSiO2 have been prepared by solid-state reactions. The phase composition and thermodynamic stability of the composites have been demonstrated by X-ray diffraction and thermal analysis (thermogravimetry and differential scanning calorimetry), respectively. The morphology of the composites has been examined by scanning electron microscopy in combination with energy dispersive spectroscopy. The electrical conductivity of the composites has been measured as a function of temperature and oxygen pressure and shown to have an ionic character. Heterogeneous doping of lanthanum and samarium tungstates has been found to have opposite effects: the addition of 1–5 mol % SiO2 nanopowder to La2(WO4)3 leads to an increase in its electrical conductivity by seven times, whereas analogous heterogeneous doping of Sm2(WO4)3 reduces its electrical conductivity.