Different proportions of ammonium heptamolybdate (AHM) supported on gamma-alumina were prepared by the impregnation method. The thermal decomposition stages of AHM and the solid products were investigated using DTA, TG, XRD and IR techniques. The results obtained revealed that the absence of all peaks accompanying the decomposition stages of AHM at low loading levels (< 20 mol%) was due to the dispersion capacity of the support. On increasing the % loading of AHM, gamma-alumina retarded the formation of the two intermediates, (NH4)4Mo7O23.2H2O and (NH4)2Mo7O22.2H2O, while favoring the decomposition of the latter to produce solid MoO3. The produced MoO3 readily interacted with Al2O3 above 500-degrees-C to form Al2(MoO4)3 phase which became highly crystallized when calcined at 700-degrees-C. The aluminum molybdate spinel formed was thermally stable up to 800-degrees-C, and then decomposed above this temperature into alpha-Al2O3 and MoO3. Moreover, the Al2O3 support decreased the crystallinity of MoO3, whereas Al2(MoO4)3 only affected the surface-active site, Mo=O. Finally, the presence Of MoO3 greatly enhanced the crystallization of alumina into the kappa- and alpha-phases upon heating at 700 and 900-degrees-C, respectively.