[en] Electron ionization cross-sections for CO, CO dimers and trimers have been investigated near the threshold with a newly constructed crossed beams apparatus using a hemispherical electron monochromator (HEM) to monochromatize the primary electron beam. The dissociative attachment cross-section curve for CO used to test the new set-up shows, in accordance with the high-resolution study of Stamatovic and Schulz, a vertical onset at the thermochemical threshold (9.63 eV) of the lowest possible DA channel O^-(²P)+C(³P) in excellent agreement with the value of 9.63 eV derived from known thermochemical data, thereby indicating an energy scale accuracy of better than 10 meV. The appearance energies (AE) of some rare gases (Ar, Kr, Xe) and molecules (N₂, O₂, N₂O) measured for calibration purposes agreed with the known ionization energies (IE) of these compounds within 10 meV. Using a novel data handling procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, the following appearance energies were obtained from the measured ionization cross-section curves using the AE(CO⁺) as reference: AE((CO)₂⁺)=13.19±0.10 eV and AE((CO)₃⁺)=12.98±0.34 eV. These values are in fair agreement with ionization energies values obtained in a high-resolution photoionization experiment by Ng and co-workers yielding 13.05±0.04 and 12.91±0.04 eV, respectively. (Copyright (c) 1998 Elsevier Science B.V., Amsterdam. All rights reserved.)