A mass gain for copper nanoparticles with a mean diameter of 20 nm in a 20% oxygen-nitrogen atmosphere was measured at 150-300 degrees C using thermogravimetry (TG). The mass gain equilibrium of the copper particles differed at each temperature, and a threshold temperature was recorded. Oxide products, consisting mainly of Cu(2)O, formed on the copper nanoparticles below the threshold temperature. Above the threshold temperature, there was an initial, and drastic, formation of Cu(2)O, which then changed to CuO. The activation energy for the oxidation of the copper nanoparticles that could be calculated from the rate of the mass gain was an indication of the nano-effect of the copper nanoparticles. (C) 2011 Elsevier Ltd. All rights reserved.