The dispersion and stability of nanofluids obtained by dispersing CuO nanoparticles (obtained from different sources) in water have been analyzed. The volumetric behaviour up to high pressures (45 MPa), and atmospheric pressure viscosity were experimentally determined. It has been found that the influence of particle size in density is subtle but not negligible, but the differences in viscosity are very large and must be taken into account for any practical application. These viscosity differences can be described qualitatively by considering a theory describing either the aggregation state or the particle size distribution of the nanofluid. (C) 2010 Elsevier B.V. All rights reserved.