The collision system CCO²⁺+Ne has been studied by energy gain spectroscopy at a collision energy of 100 eV. Cross sections for capture to final CO⁺ states have been measured, and the authors show how the energy gain spectrum can give information on the state of the CO²⁺ projectiles. It is demonstrated that the present method is superior to other methods when it comes to placing level energies of CO²⁺ on an absolute scale.