Cobalt aluminate (CoAl2O4) nanopowders were synthesized by reverse microemulsion process using cyclohexane, a nonionic surfactant, and aqueous solutions of cobalt (II) nitrate hexahydrate and aluminum nitrate nonahydrate. The influence of water to surfactant molar ratio on the size of powders has been studied. The X-ray diffraction analysis shows that the powders calcined at 1000 °C have single phase cubic spinel structure. The average size of the particles increases with increasing water to surfactant molar ratio. The mean diameter of the particles varies between 42 and 57 nm. The Fourier transform infrared spectra also confirm the formation of CoAl2O4. Magnetization study reveals that the CoAl 2O4 sample exhibits superparamagnetic behavior. © 2010 Elsevier B.V. All rights reserved.