The absorption coefficients of the pure CO2 gas in the region of 17 mum were examined. A special attention was paid to the line mixing effect that influenced the spectral shape in the Q-branch regions of the absorption bands. 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; this matrix is constructed using the shape analyses of the band of a different spectral region (Edwards and Strow, 1991). The second method is based on the strong collision model with adjusted branch coupling (Tonkov et al., 1996), and it is necessary to use only one empirical parameter for all the bands to describe the line mixing effect on line shapes. This method was generalized to the bands of a PiDelta type. The merits and the demerits of these two methods are discussed, and the results of the correspondent calculations are compared to experimental shapes. It is inferred that the strong collision model for the absorption coefficient calculations can be applied in solving the non-LTE radiative transfer problem in CO2 bands in the atmosphere of Mars.