Abstract: Hypersonic technology is one of the key topics in aerospace and deep space exploration. The thermochemical nonequilibrium is one of the characteristics of hypersonic flows. CO is an important intermediate during hydrocarbon combustion and is the main radiation source during the entry processes of Mars and Venus. Studies on the CO thermal nonequilibrium effects are important to the above research. An interference–free Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique was applied to the measurements of time–dependent rovibrational temperatures and thermal nonequilibrium effects for reflected–shock–heated CO mixtures in a kinetics shock tube. The measured data were compared with predictions based on the Landau–Teller model and the Schwartz–Slawsky–Herzfeld model. The model predictions agreed well with the measured data when adopting the modified relaxation data (including the CO−Ar system and the N₂–N₂ system) in the literature. Further analysis highlighted that the vibration–vibration–translation internal energy exchange mode yields consistent CO–T_vib and N₂–T_vib. Therefore, for the 1.0%CO+99.0%N₂ mixture, the measured CO–T_vib can be used to characterize the thermal nonequilibrium effects of high–temperature N₂.