The effect of MnO 2 as an additive on the densification behaviour and microstructural evolution of α-Al 2 O 3 has been investigated over a wide sintering regime. A colloidal technique was applied for the mixing of MnO 2 in order to achieve a uniform distribution and homogeneous microstructures. Sintered density, average grain size and microstructural features were elaborated along with the microhardness and mechanical strength measurements. Manganese oxide was found to promote grain growth and an intergranular second phase formation was detected for additions over 0 5 wt% MnO 2 . Excessive grain growth and draining of the second phase were observed in the samples with 3 wt% MnO 2 additions sintered above 1500°C.