Abstract The ionization of acetonitrile has been studied using both electron and photon ionization combined with coincidence detection techniques. Relative partial ionization cross sections and relative precursor-specific partial ionization cross-sections are determined for all the cations formed from single, double and triple electron ionization of CH3CN from 30 to 200 eV electron energy. These cross sections characterize, distinguish and quantify the positive ions formed from these levels of ionization over the electron energy range investigated. To complement these electron ionization studies, relative ion yields for the formation of single ions (both monocations and dications) and monocation pairs have been measured following photoionization of CH3CN from 20 to 42 eV using synchrotron radiation. There is a strong similarity between the photoionization data and electron ionization data.