The O-exchange reaction between CO2+ and CO is triggered by two different entrance channels in C-13-labeled systems. The unimolecular dissociation of the (CCO3+)-C-13 reactive intermediate is investigated at varying energies by mass spectrometric techniques. An almost "one-way" O-exchange at energies close to the dissociation threshold discloses a remarkably large intramolecular C-12/C-13 kinetic isotope effect ranging from 6.0 to 7.3 +/- 0.5.