Gold/iron oxide core/hollow-shell composite nanoparticles (NPs) with controllable shell thicknesses are synthesized (see figure). The gap between the Au core and iron oxide shell is formed as a result of different outward and inward diffusion rates of Fe and 0, respectively. Control over interparticle interactions allows encapsulation of several Au cores inside one iron oxide shell. Superparamagnetic measurements of the NPs at room temperature demonstrate the plasmon resonance at 565 nm.