Supercritical antisolvent micronization (SAS) has been used to obtain microparticles of several kind of materials, but the production of nanoparticles have been observed and studied in some cases only. This work is focused on the systematic production of nanoparticles using SAS. We performed experiments on several compounds and different solvents at selected operating conditions, obtaining nanoparticles with mean diameters ranging between 45 and 150 nm, thus demonstrating that nanoparticles production is a general characteristic of this process. Moreover, we found a correlation between nanoparticles mean diameter and the reduced concentration of the starting liquid solution that can allow the prediction of the mean diameter obtainable at fixed process conditions. Nanoparticles with mean diameters as small as 45 nm have been obtained, operating at 150 bar, 40 degrees C and x(CO2) = 0.97; but, even smaller nanoparticles can be obtained operating at higher pressures. The mechanism that produces nanoparticles in supercritical antisolvent precipitation has also been discussed. (C) 2007 Elsevier B.V. All rights reserved.