Display omitted •Single-phase nanoparticles of CoxCe1-xO2-y exist only for x < 0.2 and their reducibility and catalytic activity increase with x.•Nanoparticles of Co3O4 are similarly active to the most active SPhSS: Co0.15Ce0.85O2-y.•Catalytic activity of Co3O4 increases with small additives of CoxCe1-xO2-y phase.•This synergy can be explained by specific activity of the interfacial areas.•Coexistence of both types nanoparticles seems to provide the highest activity. Two different nanoparticle oxide systems are obtained depended on Co:Ce atomic ratio (R) using the same reverse nanoemulsion method. The single-phase solid solutions (SPhSS) are formed up to the ratio 1:4 < R < 1:5. The higher Co concentration results in segregation of Co3O4 phase forming nanocomposite of 2 types of nanoparticles (SPhSS and Co3O4). These nanomaterials were characterized using X-ray and electron diffraction, transmission electron microscopy together with linescan EDX analysis of individual nanoparticles, high-resolution XPS, TPR of preoxidized or pre-reduced samples, and testing activity in methanol total oxidation. The catalytic activity increases with increase of Co concentration in SPhSS and is preserved after deposition on alumina support. The catalytic activity of pure Co3O4 nanoparticles is comparable with the Co0.15Ce0.8O2-y solution and is increasing with Ce doping in the Co-rich 2-phase nanoparticle systems. Such a coexistence seems to be responsible for the highest catalytic activity in methanol oxidation.