The ionization cross section of hexamethyldisiloxane (HMDSO) molecules (m/z = 162), pentamethyldisiloxane radicals (m/z = 147) and tetramethyl hydro disiloxane radicals (m/z = 133) have been measured versus electron energy up to 30 eV. The radicals are produced selectively by two different mechanisms which are the energy transfer by collision with metastable argon species and the VUV photolysis. The absolute ionization cross section values are determined and analytical functions derived from the Binary-Encounter-Bethe theory (BEB) are given to calculate the direct ionization of molecules or radicals versus electron energy lower than 30 eV. (C) 2014 Elsevier B.V. All rights reserved.