The article presents a method to synthesize aluminum oxynitride (AlON) doped with cobalt ions at concentrations ranging from 0.01 at.% to 5 at.%. The phase and elemental composition of the samples and their optical properties have been studied upon variation of the cobalt content. Five emission bands in the pulsed cathodoluminescence spectra are attributed to intrinsic defects and impurities in AlON. The optical gap of AlON:Co estimated using the Tauc method are in the range of 5.72–5.84 eV and are significantly influenced by the oxygen content in the AlON host.