A modified nanoemulsion method allowed obtaining nanoparticles (NPs) of the MnOx-CeO2 system within the whole range of the Mn:Ce atomic ratio. The NPs with excess of Ce consisted solely of MnxCe1-xO2 nanocrystallites (NCs) (fluorite structure) while the catalysts with excess of Mn were nanocomposite of MnxCe1-xO2 and of 3-4 times larger Mn3O4 (hausmannite) NCs. The obtained nanomaterials were mesoporous but the composite materials consisted thin slit-shaped pores of size close to the micropore range. The NPs of Mn3O4 were much more active in catalytic combustion of methanol than NPs of CeO2 obtained by the same nanoemulsion method. Nevertheless, incorporation of Mn into CeO2 lattice can improve these NPs to be more active than Mn3O4. The NPs of MnxCe1-xO2 deposited on alpha-Al2O3 micrograins via the nanoemulsion method preserved activity of the equivalent non-supported NPs. The comparable high activity of Mn0.25Ce0.75O2 monophasic NPs with the nanocomposite of the Mn:Ce = 3:1 ratio suggests joint catalytic action of Mn- and Ce-assembles. Complementary XPS measurements showed a surface Mn-enrichment of the monophasic NPs associated with Mn4+ to Mn2+/3+ reduction and increase of the O2- share in O 1s signal. Such active assemblies have been postulated. (C) 2015 Elsevier B.V. All rights reserved.