Symmetrical tilt grain boundaries (STGBs) on the (10 (1) over bar0), ((2) over bar 112), and (01 (1) over bar8) planes in alpha-Al2O3 have been investigated for their behavior with respect to yttrium doping by performing a combined study of high-resolution transmission electron microscopy and spatially resolved energy dispersive X-ray analysis. Bicrystals have been produced by diffusion bonding under ultrahigh vacuum; yttrium was introduced before the bonding process. The prismatic twin has a bulklike grain boundary (GB) structure and does not accommodate any yttrium at the GB. The yttrium at the Sigma17 ((2) over bar 112) GB and the Sigma37 (01 (1) over bar8) GB changes the GB structure and the content of other impurities.