Experimental and theoretical results on the influence of line mixing on the shape of infrared CO2 Q branches of importance for atmospheric applications are presented. Two Q branches of Sigma <-- Pi symmetry, which lie near 618 and 720 cm(-1) and belong to the 10(0)0(II)<-- 01(1)0(I) and 10(0)0(I) <--01(1)0(I) bands, have been studied for many conditions of temperature (200-300 K), total pressure (0.5-10 atm), and mixture (with He, Ar, O-2, and N-2) The theoretical approach used is based on the Energy Corrected Sudden approximation; its parameters have been deduced from both line-broadening data and measured absorption by the Q branches. Comparisons between experimental and computed spectra demonstrate the quality of the model, regardless of the conditions. Detailed analysis of the influences of the Q-lines spectral spacing, temperature, total pressure, and collision partner are presented. They show that significantly larger line-mixing effects are obtained when CO2-He is considered with respect to CO2-(Ar,O-2,N-2) This is analyzed in terms of the relative contributions of the short-and midrange interaction forces and of propensity rules resulting from the coupling of angular momenta. (C) 1997 American Institute of Physics.