Abstract The nascent transient linewidths of the CO 2 00 01, J→00 02, J−1 transitions were measured immediately following collisions with hot H and D atoms of 2.30 and 2.16 eV energy, respectively. All lineshapes were well fitted by Gaussian profiles, but were significantly broader than the room-temperature Doppler profiles. D atom collisions resulted in broader linewidths than H atom collisions. The observed widths are in good agreement with a simple billiard ball model for a specific rotational level J. However, the linewidth was observed to increase with increasing rotational state within the 00 01 manifold for both hot H and hot D atom collisions.