The effect of MgO-doping (2-10 mol%) on solid-solid interactions and phase transformation process in MoO3/Al2O3 system were studied using thermogravimetry and X-ray diffraction (TG, DTG and XRD) techniques. The proportions of molybdena expressed as weight percent were 12.36, 22.01 and 41.37. The results obtained showed that the MgO-doping promoted the solid-solid interaction taking place at 500 degrees C between Al2O3 and MoO3 to produce Al-2(MoO4)(3). However, MgO interacts readily with MoO3 at temperatures starting from 500 degrees C yielding MgMoO4 which remains stable even when heating at 1000 degrees C, The produced Al-2(MoO4)3 decomposed at temperatures starting from 800 degrees C producing alpha-Al2O3 and MoO3, a portion of which sublimed and the other portion dissolved in alumina matrix forming MoO3-Al2O3 solid solution. MgO-doping decreased slightly the solubility of MoO3 in Al2O3, The promotion of Al-2(MoO4)3 formation at 500 degrees C and the decrease in the solubility of MoO3 in Al2O3 by MgO have been attributed to dissolution of a small portion of MgO in the MoO3 lattice with subsequent increase in the mobility of Mo6+ ions. The promotion effect of magnesia towards Al-2(MoO4)3 formation and dissolution of MoO3 in Al2O3 solid are relatively small when compared to the observed effects reported in the case of Li2O-doping of MoO3/Al2O3 system. The limited effect of MgO-doping has been attributed to a limited solubility of MgO in MoO3/Al2O3 system due to the formation of MgMoO4.