alpha-Alumina films formed by anodic spark deposition in the melt of NaHSO4-KHSO4 mixture (200-degrees-C) exhibited a porous structure. It has been found that the porosity and amount of alpha phase varied with anodic current density. By electrical measurements, films were found to exhibit a continuous open-pore structure, causing failure of humidity sensors. This problem can be overcome by reanodization of the anodic-spark-deposited alumina films in either borax aqueous solution or sulfuric acid solution for a short time to form an effective barrier layer at the pore base. The modified anodic-spark-deposited alumina films showed highly sensitive response to water vapor change in the range of -76-degrees to +20-degrees-C dew point with much improved long-term stability. The humidity characteristics of the alpha-alumina films were qualitatively explained in terms of the BET theory and condensation theory.