Low-temperature route to pure CoO nanoparticles based on the oxidative decarbonylation of dicobalt octacarbonyl promoted by amine oxides is presented. The small size of the resulting particles (1.7 nm) leads to spontaneous nanoparticle aggregation. Magnetic measurements show a bimodal behavior with two distinct blocking temperatures, which can be ascribed to the individual CoO nanoparticles and to narrowly size-dispersed aggregates.