The redox process of Co^+z ions (z=2,3) substituted into the sapphire alpha -Al₂O₃ structure produced by heat treatment in H₂ and O₂ atmosphere is correlated with ESR and ENDOR spectra of (Co⁺², H⁾) and with UV-VIS and IR spectra of Co⁺³: alpha -Al₂O₃ and (Co⁺², H⁺): alpha -Al₂O₃ respectively. From ENDOR spectra the charge compensator H⁺ for Co⁺²(c): alpha -Al₂O₃ is uniquely identified and the reduction of the crystal-field symmetry by it from D_3d to C₁ is demonstrated. Location of the charge compensator and structural parameters of the defect (Co⁺²,H⁺): alpha -Al₂O₃ are determined. Furthermore, it is shown that the proton-ENDOR spectra of (Co⁺², H⁺): alpha -Al₂O₃ are described by a hyperfine interaction term characterised by an asymmetric coupling tensor originating from dipole-dipole interaction.