The electric and magnetic properties as well as the thermal stability of CoxFe3-xO4 nanopowders, (0.6 < x < 1.8) were investigated. These powders were synthezised using a one pot solvo-thermal route with acetylacetonates as precursors. The properties were linked to the size, morphology, composition of the particles and to the cation distribution. With the exception of x = 0.6, the powders are stable up to 600 degrees C. Whatever the composition, Co2+ has a strong tendency to occupy tetrahedral sites, contrary to what occurs in bulk ferrites. The nanopowders display a semi conducting behaviour. Between ambient and 500 degrees C, conduction occurs between Co2+ <-> Fe3+ pairs, and intergrain conduction predominates. The conductivity is in the 10(-7)Omega(-1) cm(-1) range. The CoxFe3-xO4 nanopowders behave magnetically as a superparamagnetic assembly of single-domain particles. The magnetocrystalline anisotropy constant is significantly higher for these nanoparticles than for bulk ferrites. Co1.8Fe1.2O4 displays the lowest blocking temperature (200K) and the highest anisotropy (K= 21 x 10(6) erg/cm(3)). (C) 2014 Elsevier Ltd. All rights reserved.