Torr. Briefly, a Ta(110) substrate was cleaned by heating to 2100 K for 5 minutes, then alumina was deposited by evaporating Al in 5 x 10 -6 Torr of O2 at a surface temperature of 970 K. The alumina films were grown to a thickness of ~4 nm, as determined by XPS, based on previous work showing that for alumina films thicker than ~3 nm, the chemistry of supported clusters is thickness independent. Previous work has shown that films grown under these conditions have hexagonal symmetry, ISS showed no evidence of surface Ta for alumina films in this thickness range on Ta(110), and CO TPD (Fig. ) from the alumina and Ge/alumina films showed no evidence of the strong binding sites expected for Ta. Thus we conclude that the alumina films are solid, exposing only Al and O binding sites.