The infrared absorption spectrum of thick samples of solid CO2 (~4 mm path length) has been investigated in the frequency range 1100–1600 cm−1 at temperatures from 88 to 160 K. The (ν1, 2ν2) Fermi doublet is inactive because of the crystal symmetry but absorption is observed with maxima at the frequencies ν1 ± ai, 2ν2 ± ai(i = 1, 2, 3, 4). At 88 K the values of ai are 60, 75, 95, and 108 cm−1; at lower temperatures the maxima are sharper and the ai greater. These four frequencies correspond to maxima in the phonon frequency distribution. The agreement with existing theoretical calculations is not good; there is better agreement with the zone boundary frequencies determined for two symmetry directions by inelastic neutron scattering experiments. The present results are consistent with previously observed combinations with the allowed ν2 and ν3 internal modes.