Air-stable high temperature solar selective surfaces have the advantages of simplifying the design, and reducing the cost of solar thermal energy conversion systems. Previous studies on the properties of titanium nitride (TiN) or titanium-aluminum nitride (TiAlN) films suggested that these materials could be a candidate for solar energy applications. In this paper, we report that oxidation occurs at 450 degrees C, and an oxide layer of about 2030 nm was formed after only a few minutes of heat treatment with oxygen. The thickness of the oxide layer is comparable to the thickness of the absorbing layer of the solar thermal selective absorbers, which can affect significantly the solar thermal performance. TiN produced at higher nitrogen pressure (2.1 Pa with 40% nitrogen in argon) could absorb oxygen more easily into bulk and was less oxidation resistant during the heat treatment than that produced at 0.4 Pa of 40% nitrogen in argon. The hardness after the oxidation treatment was slightly increased by approximately 10%, consistent with reported oxidation resistant properties of this material for mechanical protection applications. As a result of this study, TiN or TiAlN as an element may not be suitable candidates for use as solar selective absorbers in air-stable high temperature applications. (c) 2006 Elsevier B.V. All rights reserved.