Infrared absorption measurements have been carried out on a carbon–oxygen complex in gallium arsenide. The stretching mode at 2059.6 cm−1 has been investigated at low temperatures with polarized light under uniaxial stress in the 〈100〉, 〈111〉, and 〈110〉 crystallographic directions. The splitting behavior due to lifting of orientational degeneracy shows that the center has tetragonal symmetry. The intensity of the isotope shifted satellite band caused by the naturally abundant 18O isotope is not in agreement with the identification as a simple CO-molecule. The number of incorporated oxygen atoms is 3 ± 1. The assignment of the defect to a CO2 molecule is in agreement with the experimental data available so far.