Sintered Al(2)O(3) of high Si content and various grain diameters were investigated by Positron Annihilation Lifetime Spectroscopy at room temperature. Two spectrum components of lifetimes 137 +/- 1 ps and about 400 ps were resolved. The shorter is assigned to Al vacancies, V(Al)(n), induced by Si dissolution and located within the grain, while the other to grain boundary clusters. A linear correlation between the intensity of the longer component and the specific surface of grain is found, which somewhat strengthens the assignment of this component. The results are discussed in light of the previous ones dealing with Al(2)O(3) of low silicon content.