Thin films of mixtures of molybdenum silicate (MoSi2) and silicon (Si) (MoSiX, where the Mo to Si molar ratio = 1:X) were deposited on silicon nitride (Si3N4) polycrystalline substrates by radio-frequency magnetron sputtering using a target made of a mixture of MoSi2 and Si powders. The crystal structure of MoSiX thin films deposited on the Si3N4 substrate consisted of a mixture of a hexagonal phase and an unknown phase when X > 2.05. A thin film consisting almost entirely of the unknown phase could be deposited when X = 2.1−2.15. Molybdenum silicate can exist in the forms Mo3Si, Mo5Si3, or MoSi2, but to date there has been no report of molybdenum silicate having a Si to Mo molar composition ratio of larger than 2. It was found that the surfaces of thin films of the hexagonal phase or the unknown phase were readily oxidized, whereas the surfaces of thin films of a mixture of the hexagonal phase and the unknown phase exhibit excellent oxidation resistance in air at temperatures up to 700 °C.