TiN was introduced as a part of the sensing layer of gas sensitive metal-insulator-semiconductor (MIS) devices. Three types of metallic gate layer structures deposited by magnetron sputtering were investigated: TiN, a double layer with Pt on top of TiN, and two-phase Pt-TiN films formed by co-sputtering. The homogeneity of the co-sputtered layer was strongly dependent on the substrate temperature during film growth, with segregation of Pt as a result of high temperature deposition. During the deposition conditions in this work, Pt and TiN appear to be immiscible, resulting in growth of films consisting of the two phases. Furthermore, surface oxidation of TiN and enhanced oxidation of TiN at the grain boundaries to Pt in both the as-deposited films after exposure to atmosphere at room temperature and the films subjected to MIS device processing and to gas response analyses at a temperature of 140degreesC resulted in a three-phase TiN-TiOx-Pt system. A segregation of Pt to the growth surface was observed during co-sputtering at 900degreesC, but not at 400 degreesC. The gas response to hydrogen, ammonia, propene, and acetaldehyde was measured and it was found that devices containing Pt were sensitive to all test gases. Devices with pure TiN, showed no response to any of the test gases, and the implication of that result, on the gas response mechanisms, particularly for ammonia, is discussed.