The surface properties of a Ba-beta-alumina sample (Ba : Al ratio 1 : 12) very stable to sintering (15 m(2)/g after calcination at 1670 K for 10 h) have been investigated by FT-IR spectroscopy of adsorbed probe molecules (water, pyridine and CO2) and by measuring the isoelectric point. The surface has a medium-strong basic character,justified by the preferential exposition of Ba-O-containing "mirror planes", parallel to 0001 planes, on the surface of plate-like particles. This behavior has been associated to the resistance to sintering, typical of this material, possibly originated by the difficulty of the very big Ba2+ ions to migrate into the spinel-type alumina blocks. This should imply the blocking of the crystal growth along the crystallographic c axis.