In the Hg-flash-photosensitized reaction of carbon monoxide, CO(v5) was detected by kinetic absorption spectroscopy in the vacuum ultraviolet. Even with [CO] < 1 Torr and with short delay times, the ratio [CO(v= 1)]:[CO(v= 2)] was always high. By analyzing the effects of vibrational exchange processes, it was demonstrated that the CO(v= 1) largely arises from the spin-orbit relaxation Hg(63P1)+ CO(v= 0)→ Hg(63P0)+ CO(v= 1) which produces vibrationally excited molecules with approximately unit efficiency. It was shown that there is negligible quenching of Hg(63P1) by Ar at pressures up to 500 Torr.