We observe the spontaneous formation of hollow cobalt oxide nanoparticles at room temperature, indicating an enhancement of the solid-state diffusion at the nanoscale. Single crystal cobalt nanoparticles covered by a hydrophobic organic layer were transformed spontaneously into CoO hollow nanoparticles when deposited on the water-air interface in a matter of a few hours. The presence of water modifies the reactivity on the nanoparticle surface favoring the formation of the hollow structure; otherwise Co-CoO core-shell nanoparticles are obtained. The CoO hollow nanoparticles are formed only in an intermediate state, and after a period of time these structures finally undergo disintegration to form minor CoO entities. © 2013 The Royal Society of Chemistry.