Several low-index symmetrical tilt grain boundaries in alpha-Al2O3 were investigated by combining high-resolution transmission electron microscopy and spatially resolved energy dispersive X-ray analysis. Bicrystals with a well-defined orientation relation and interface plane were produced by diffusion bonding under ultrahigh vacuum. Grain boundaries like the prismatic and rhombohedral twin develop an atomically sharp and clean interface, whereas higher index grain boundary (GB) planes show a more complex structure and are able to accommodate both Si and Ca in the interface. Additionally, it was found that asymmetric GB planes which occur as facets at rhombohedral and prismatic twins also accommodate impurity atoms.