The production of charged nanoparticles from hot surfaces like glowing wires was used earlier in connection with mobility classification, but the phenomenon was not understood. In the present paper, negative particle charge is explained by thermoemission of electrons and positive particle charge by surface ionization of impurity atoms with low ionization energy. Such impurities are inevitably present even in high-purity metals. A model based on Saha-Langmuir and Richardson equations for particle charging and Hertz-Knutsen and Clausius-Clapeyron equations for evaporation rate can be fitted to the experimental charge emission in a satisfactory manner. This applies to the charge attached to Ag and Pd nanoparticles forming from a glowing wire (this work) and to data on charge emission from hot surfaces in vacuum reported by Jeon et al. The charging probability of particles is proportional to their volume indicating that the charge was incorporated during the formation process. (c) 2007 Elsevier Ltd. All rights reserved.