This Paper presents the simulations of CO 2 and CO infrared radiation behind shock waves and in expansion flows. This work is motivated by the little amount of characterizations of CO 2 infrared radiation in these expansion flows and thermodynamic regimes, whereas carbon dioxide infrared radiation is deemed to contribute significantly to the afterbody heating withstood by a Martian spacecraft. Japan Aerospace Exploration Agency (JAXA) shock-tube facilities were operated at conditions generating flow velocities ranging from 2.8 to 7.3 km∕s. Absolute infrared spectra of CO 2 and CO were obtained under equilibrium and nonequilibrium conditions and used to assess the performances of the spectral solvers. Specifically, this Paper introduces the recent enhancements brought to JAXA in-house codes to compute CO 2 nonequilibrium infrared radiation. Schemes to split the level energy into its pure vibrational, rotational, and vibration-rotation coupling and interactions contributions are proposed and implemented. The implementation was evaluated with comparison with literature data and recent measurements. Infrared radiation behind shock waves and under expansion regimes was analyzed with a newly developed model to infer the vibrational and rotational temperatures and determine the degree of thermal nonequilibrium. The latter outlines the highly complicated vibrational energy distribution of carbon dioxide during expansion and the subsequent radiation, as well as the possible shortcomings of the multitemperature models. Therefore, these studies of carbon dioxide nonequilibrium radiation during expansion pose new challenges to the community and will enable the upgrade and validation of the multitemperature and state-to-state models currently in use.