A density matrix formulation of the interaction of radiation with bands of pressure‐broadened overlapping spectral lines is described which includes the effects of interaction between neighboring lines due to collisions. A measurement of the abosrption coefficients at the centers of several lines of the 10.6‐ and 9.4‐&mgr;m bands of CO2for CO2and mixtures of CO2and N2and He at absolute pressures to 18 atm is reported. The calculated spectra of these bands are given for the two limiting forms of overlapping line theory: one which maximizes line interaction and another which neglects it, calculating the spectra as sums of Lorentzian‐shaped lines. The data are shown to be in good agreement with the theory which maximizes the line interaction.