The effect of small additions of silica (SiO2) on the densification and microstructural evolution of an ultrapure α-alumina (Al2O3) has been investigated. The role of SiO2 on the sintering behaviour was elucidated through an extensive dilatometric study performed on pure alumina and on doped samples with small amounts of colloidal silica (up to 2000 ppm). It has been found that the addition of SiO2 leads to a significant decrease of the shrinkage rate at the intermediate state of sintering due to the reduction of point defects (predominantly oxygen vacancies). Doping with small amounts of SiO2 results in heterogeneous microstructures in which abnormal grain growth was observed above a critical temperature of near 1600 °C. Below this temperature, silicon was detected at the grain-boundary triple junctions forming amorphous pockets and over 10 nm at each side of the grain boundaries. Above 1600 °C, a glassy phase of 2-3 nm in thickness was formed at the grain boundaries. © 2007 Elsevier Ltd. All rights reserved.