The IR absorption of the pure CO2 gas in the region of 15 mum was examined. A special attention was given to the line mixing effect that influences the spectral shape of the vibration-rotation absorption bands in the Q-branch regions. Two methods in shape description were analyzed. The first method uses the Rosenkranz line shapes with the line mixing parameters, which are found from an empirical rotational relaxation matrix. The second method is based on the strong collision model with adjusted branch coupling (ABC-model). The merits and the demerits of these two methods are discussed, and the results of the corresponding calculations are compared to the measured shapes. It is inferred that the ABC-model for the absorption coefficient calculations can be successfully applied for solving the non-LTE radiative transfer problem in CO2 bands in the atmospheres of Earth-like planets. (C) 2001 Elsevier Science Ltd. All rights reserved.