Monodisperse ligand-capped cubic wüstite FexO nanocrystals were prepared by a novel thermal decomposition method of iron (II) acetate in the presence of oleic acid as the surfactant. Controlled size distributions of cubic nanoparticles possessing the rock salt crystal structure were isolated in the range 10–18 nm. The influence of molar ratio of surfactant to precursor was investigated to understand size control and monodispersity. Using inexpensive, nontoxic metal salts as reactants, we were able to synthesize gram-scale quantities of relatively monodisperse nanocrystals in a single reaction, without further size selection, characterized by x-ray diffraction and transmission electron microscopy. The procedure enables the collection of samples of uniform size as a function of time, thus permitting a preliminary solid-state kinetic analysis of the reaction as a function of increasing particle size. Following controlled evaporation from nonpolar solvents, self-assembly into two-dimensional arrays, three-dimensional single-component superlattices, and binary superlattices with gold nanoparticles were observed and characterized.