C-axis compressive strengths of pure and titanium-doped sapphire were measured at 600 degreesC in air. Single crystal sapphire was doped with both titanium (3+) and titanium (4+) ions from mixtures initially containing 0.05% wt. to 0.25% wt. Ti2O3 in Al2O3. It was shown that both valence state and concentration of titanium were important in slowing down twin boundary movement under c-axis compression. Titanium (4+) doping did not adversely affect the thermal conductivity and infrared optical properties of sapphire.