Abstract. The magnetic circular X-ray dichroism (MCXD) of CoO nanoparticles was measured at low temperatures and in high magnetic fields. The particles show a superparamagnetic behaviour at room temperature, and a large orbital contribution to the magnetic moment at low temperatures was observed. This enhancement of orbital magnetism is largely determined by the contribution of the surface atoms. The larger spin-orbit coupling of the electrons of the surface atoms of the particle can cause a change in magnetic ordering in the core of the particles.