Room-temperature Ba deposition on an oxygen-terminated theta-Al2O3/NiAl(100) ultrathin film substrate under ultrahigh vacuum (UHV) conditions is studied using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) techniques. In addition, Ba oxidation by the ions of the alumina substrate at 300 K < T < 1200 K in the absence of a gas-phase oxidizing agent is investigated. Our results indicate that at room temperature Ba grows in a layer-by-layer fashion for the first two layers, and Ba is partially oxidized. Annealing at T < 700 K results in further oxidation of the Ba species, whereas annealing at higher temperatures leads to loss of Ba from the surface via desorption and subsurface diffusion.