Abstract The problem of the pressure broadening of spectral lines is investigated with respect to the microwave absorption of two terrestrial atmospheric gases, oxygen and water vapor. A formalism and accompanying computational procedures under the impact approximation and using the Liouville (line) Space concept introduced by Baranger and elaborated by Fano are employed. It is desired that the theory would obviate the necessity for specific line shapes, provide a means of computing line widths and account for line overlapping effects from first principles. The present approach is found to be generally adequate for the treatment of O 2 absorption on comparison with experimental data for line widths, absorption coefficients and zenity opacities. While H 2O line widths are satisfactorily predicted, estimated resonance overlapping effects are found to be far from sufficient (accounting for at most ~ 5%) to explain the theoretically controversial anomalous absorption.