Absolute partial cross sections for electron-impactionization of CO are reported for electron energies from threshold to1000 eV. The product ions are mass analysed using a time-of-flight massspectrometer and detected with a position-sensitive detector whoseoutput unequivocally demonstrates that the various product ions arecollected with equal efficiency irrespective of their initial kineticenergies. Data are presented for the production of CO+, C+, O+and CO2+, and for the total cross section which is obtained as the sumof these partial cross sections. The overall uncertainty in the absolutecross section values is ±5% for singly charged parent ions and±6% for fragment ions. Comparison is made with prior experimentsand calculations.