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Water adsorption and surface conductivity on ceramic substrates are important parameters in the electrochemical corrosion of thin-film metallizations and the associated growth of dendrites in humid environments. The adsorption of water provides the necessary electrolyte through which ion migration and electrochemical corrosion is possible, and the surface conductivity of the adsorbed moisture layer is a direct measure of the ion migration capability. The water adsorption isotherm of or-alumina at 23°C was determined using thermo-gravimetric analysis. It is found to be of type III according to Brunauer's classification. Similar to results for other oxides, the water adsorption on -alumina is much lower than that on metal surfaces. An equivalent of one monolayer of water is adsorbed when the relative humidity (RH) is about 35 percent. For RH's higher than 70 percent, water adsorption is more than five monolayers and increases dramatically with increasing water vapor pressure. The surface conductivity was measured using an interdigitated thin-film circuit of pure copper sputtered on an -alumina substrate. The results show that the conductivity of the water layer increases with the accumulation of water monolayers. A moisture film equivalent to less than five monolayers (RH less than about 70 percent) has a conductivity approximately two orders of magnitude lower than that of bulk water. At 95 percent RH, a water film of more than 20 monolayers has a conductivity which is still an order of magnitude lower than that of an equivalent film of bulk water. These results are attributed to a discontinuous or patchy nature of the water film on the surface of the substrate.