Shaping up for conversion: Monodisperse FeO nanoparticles were synthesized by reductive decomposition of iron(III) acetylacetonate with oleic acid (OA) and oleylamine (OAm). The nanoparticle sizes were tunable from 14 to 100 nm and their shapes were controlled to be either spherical or truncated octahedral (see images). The FeO nanoparticles were converted into diverse Fex Oy nanoparticles, which have potential in magnetic and catalytic applications.