Monodispersed 2.4 nm-sized SnO2 and 2.6 nm-sized CeO2 nanoparticles have been reproducibly formed through a non-hydrolytic solvothermal reaction with 2-ethylhexanoate complexes, without the addition of extra surfactants. During the synthesis, the dissociated 2-ethylhexanoates from the central metals work as a capping agent, and this induces size control of the nanoparticles and the suppression of interparticular aggregation. The as-prepared nanoparticles are soluble in a non-polar solvents, and by converting the capping group to a citrate, they also form transparent suspensions in an aqueous solution. The as-synthesized SnO2 nanoparticles are of the pure cassiterite structure, while those of the CeO2 are of the calcium fluoride structure. X-Ray photoelectron spectroscopy (XPS) analysis indicates that the oxidation state of the Sn and Ce are close to +4. The band gaps of SnO2 and CeO2 nanoparticles are 4.14 and 3.68 eV, respectively, which are relatively large values because of the quantum size effect.