Cobalt/magnesium mixed oxide solids and cobalt-molybdenum/magnesium mixed oxide solids were prepared by thermal decomposition of basic magnesium carbonate pretreated with different proportions of cobalt nitrate and then with calculated amounts of ammonium molybdate. The proportions of cobalt expressed as Co3O4 were 0.1, 0.2 and 0.3 mol while the concentrations of molybdenum expressed as mol% MoO3 were 2.5 and 5.0. The prepared mixed solid specimens were calcined in air at 400-1000degreesC. The solid-solid interactions in Co3O4-MoO3 were investigated using DTA, TG and X-ray powder diffraction (XRD) techniques. The results obtained revealed that MgO dissolved cobalt oxide in its lattice forming CoO-MgO solid solution. The amount of cobalt dissolved increases by increasing the temperature in the range 800-1000degreesC. This finding was confirmed by X-ray diffractograms in which all the diffraction lines of cobalt oxide disappeared at 1000degreesC. MoO3 present interacted readily with MgO and cobalt oxide by heat treatment at temperature starting from 400degreesC producing MgMoO4 and CoMoO4 which remained stable by heating at 1000degreesC. The impregnation of basic magnesium carbonate with cobalt nitrate much enhanced its thermal decomposition yielding MgO, which decomposed completely at 395.5degreesC instead of 525degreesC. The formation of magnesium cobaltite (MgCo2O4) has been ruled out via XRD investigation at relatively high diffraction angles.