By using first-principles calculations, several SO2 complexes in CdTe were studied. Based on experimental observation, SO2 complexes have been recently proposed by Lavrov et al. [Phys. Rev. B. 84, 233201 (2011)] to be the cause of the observed IR absorption peaks at 1096.8 and 1108.4 cm−1 in O-doped CdTe. Chen et al. [Phys. Rev. Lett. 96, 035508 (2006)] were originally proposed that the peaks come from OTe-V Cd complex. Our calculations indicate that the SO2 molecule on the Te site [(SO2)Te] has a low formation energy but its calculated vibration frequencies (∼900 cm−1) are lower than the observed IR modes. However, (SO2)Te can form a complex with V Cd with two possible configurations that give the vibration frequencies in a good agreement with the two observed IR peaks. The binding energies of the complex in these two configurations are about 1 eV under p-type conditions; indicating that the complex is quite stable. The two configurations are related to each other by a rotation of the SO2 molecule with an energy barrier of ∼0.4 eV. Therefore, the two configurations can co-exist at a low temperature and the high energy one gradually transforms to the low energy one as temperature increases. This agrees with the experimental observation that, at a high temperature, the two IR modes merged into one.