Dissociative ionization of hexamethyldisiloxane (HMDSO) by electron impact is investigated using Fourier transform mass spectrometry. Absolute total and partial ionization cross sections are measured and the results compared with previous studies. The collision-induced dissociation technique is used to probe the fragmentation pathways of ions, providing insight into the production of neutral products from the electron impact ionization. Ion-molecule reactions of Ar+ and HMDSO ions with the parent gas molecule are studied, and isotope studies are used to evaluate the possible reaction mechanisms. (c) 2005 American Vacuum Society.