The effects of sodium oxide doping (2-10 mol%) on solid-solid interactions and phase transformation in MoO3/Al2O3 system were investigated using X-ray diffraction technique. The proportions of molybdena were 12.4, 22 and 41.4 wt.% MoO3 and the various mixed solids were calcined in air in the 500-900 degrees C temperature range. The results obtained revealed that MoO3 interacts with Al2O3 at temperatures starting from 500 degrees C to produce orthorhombic Al-2(MoO4)3, and the completion of this reaction requires the heating of mixed oxide solids at temperatures >500 degrees C or doping with suitable amount of sodium oxide, followed by heating at 500 degrees C. The stimulation effect of Na2O in aluminium molybdate formation was attributed to a possible increase in the mobility of hexavalent molybdenum ions in MoO3 lattice. The heating of different pure and doped mixed solids (2 mol% Na2O) at 700 degrees C resulted in a complete conversion of MoO3 into aluminium molybdate and a mixture of gamma-, theta- and kappa-aluminas. The increase in the amount of dopant to 4-10 mol% brought about the conversion of aluminium molybdate into sodium molybdate, having the formulae Na2Mo3O8, Na2Mo2O7, Na2MO4O13, Na2Mo3O4 and Na2MoO4. The abundance of each compound depends, mainly, on the dopant concentration and MoO3 content present in each mixed solids specimen. Heating of pure and mixed solid sample doped with 2 mol% Na2O at 900 degrees C led to a complete thermal decomposition of Al-2(MoO4)(3) into alpha-Al2O3 (corundum) and MoO3, a portion of which sublimes and the other portion dissolves in the alumina matrix forming MoO3-Al2O3 solid solution. MoO3 much enhances the crystallization of alumina into the alpha-phase by heating at 900 degrees C instead of 1400 degrees C in the case of pure Al2O3 The doping of different MoO3/Al2O3 specimens with 4 or 10 mol% Na2O and heating at 900 degrees C increases the thermal stability of MoO3 and aluminium molybdate even by heating at 900 degrees C.