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The reduction of Co-based oxides doped with Al3+ ions has been studied using in situ XRD and TPR techniques. Al3+-modified Co3O4 oxides with the Al mole fraction Al/(Co + Al) = 1/6; 1/7.5 were prepared via coprecipitation, with further calcination at 500 and 850 °C. Using XRD and HAADF-STEM combined with EDS element mapping, the Al3+ cations were dissolved in the Co3O4 lattice; however, the cation distribution differed and depended on the calcination temperature. Heating at 500 °C led to the formation of an inhomogeneous (Co,Al)3O4 solid solution; further treatment at 850 °C provoked the partial decomposition of mixed Co-Al oxides and the formation of particles with an Al-depleted interior and Al-enriched surface. It has been shown that the reduction of cobalt oxide by hydrogen occurs via the following transformations: (Co,Al)3O4 → (Co,Al)O → Co. Depending on the Al distribution, the course of reduction changes. In the case of the inhomogeneous (Co,Al)3O4 solid solution, Al stabilizes intermediate Co(II)-Al(III) oxides during reduction. When Al3+ ions are predominantly on the surface of the Co3O4 particles, the intermediate compound consists of Al-depleted and Al-enriched Co(II)-Al(III) oxides, which are reduced independently. Different distributions of elemental Co and Al in mixed oxides simulate different types of the interaction phase in Co3O4/γ-Al2O3-supported catalysts. These changes in the reduction properties can significantly affect the state of an active component of the Co-based catalysts. [ABSTRACT FROM AUTHOR]