A sulfur-oxygen complex in CdTe characterized by IR absorption lines at 1097 and 1108 cm−1 is studied by means of infrared absorption spectroscopy and first principles theory. Temperature-sensitive measurements performed on samples with different 16O: 18O isotope ratios show that these lines are due to a split ν3 stretch vibrational mode of a disturbed sulfate ion (SO2−4 ) transformed under the A1 (1097 cm−1) and E (1108 cm−1) representations of the C3v point group. The ν1 symmetric stretch, ν4 bend, and ν1 + ν3 combination modes of the ion are also identified. An axially disturbed SO4 located at the Cd vacancy site is proposed as a microscopic model of the complex.