The absorption spectra in a range of VIS to NIR and the stretching infrared spectra of OH-radical in Ti-doped a-Al2O3 crystal are measured in a number of samples. The obtained experimental data support that the residual infrared absorption is really due to Ti3+-Ti4+ pairs rather than structure defects with adjacent Ti4+ ions in the crystal. We conclude that Ti3+-Ti4+ pairs probably locate at phase boundary between matrix and heterogeneous phase (TiO2) or in body of heterogeneous phase, i.e., the residual infrared absorption is absorption of the Ti3+ ions located at the phase boundary or in body of heterogeneous phase.