The recent development of spectroscopic techniques for extreme infrared frequencies, below 100 cm⁻¹, has enabled us to observe collision-induced absorption in non-polar gases, viz. oxygen, nitrogen, air, and helium–argon mixtures. This absorption results from the modulation of the distortion dipole moment of each pair of colliding molecules by their rotational and translational motions. It is of the kind observed previously at higher frequencies by Kiss, Gush, and Welsh using hydrogen gas. By making measurements at pressures up to 120 atmospheres, the absorption coefficients have been determined over the frequency range 30–100 cm⁻¹. Attention is drawn to the possibilities that such processes may play a role in atmospheric energy balance and might give rise to significant attenuation if beams of infrared radiation were used for communications.