Earlier thermal conductivity measurements of Mg:Al₂O₃ showed the presence of strong resonant phonon scattering, although positive identification of the scattering centre was not possible. This paper describes further thermal conductivity measurements of Al₂O₃ specimens doped with Mg, Cr and Fe in various combinations in an attempt to identify this scattering centre. The results are shown to be consistent with the theoretical predictions for Cr⁵⁺, and thermally detected EPR measurements on the samples indicate that Cr⁵⁺ is indeed present in the samples exhibiting phonon scattering. The thermal conductivity analysis indicates that three transitions within the low-lying energy-level system of Cr⁵⁺ are important.