An electron bombardment ionizer for accelerated molecular beams is described which utilizes the electron space charge potential well to suppress ions formed from molecules with thermal energies. Ions produced from molecular beams that have been aerodynamically accelerated in seeded supersonic jets to kinetic energies above 2 eV have been separated from ions formed from the propellant light molecules and background gas. The efficiency of the ionizer is approximately a factor of 10 lower than that of conventional high efficiency electron bombardment ionizers.