Line mixing in CO2 Q branches is studied in the 13.2 μm region from balloon-borne atmospheric transmission measurements. Eighty-five spectra were recorded (at 67 ¿N, 22 ¿E) in the 660--960 cm-1 region using a Fourier transform spectrometer with an unapodized resolution (full width at half maximum) of 0.013 cm-1. They involve balloon altitudes and tangent heights from 13 to 30 km and a large range of optical thicknesses. The results of computations assuming Voigt line shapes as well as accounting for line mixing are compared with measured transmissions in three CO2 Q branches of different intensities and symmetries. They confirm the significant effect of line mixing and validate recent line coupling models. As a consequence of the accuracy of the forward approach the Q branches can be used for remote sensing; this should lead to improved precision of pressure/temperature retrievals since absorption in the Q branch wing region is particularly pressure sensitive.¿ 1997 American Geophysical Union