Oxide systems prepared by thermal decomposition of mixtures of ammonium heptamolybdate with varying amounts of aluminum, gallium, and yttrium nitrates have been studied by thermal analysis, X-ray powder diffraction, and scanning electron microscopy. The formation processes, the nature of interaction between the components, and the states of these systems have been determined. It was shown that the nature of oxides in the molybdenum systems have a significant effect on their physicochemical properties.