X-ray-amorphous films 210 nm thick of Ir18Si15O67 were deposited by reactive sputtering of a target of iridium and silicon on substrates of oxidized silicon wafers, and subsequently annealed for I h at 700 degreesC in dry oxygen at pressures of 0.2, 8.0 or 760 torr. Regardless of oxygen pressure the films' composition is preserved, their microstructure changes little, and their resistivity drops only slightly from similar to1.9 to similar to1.7 mOmegacm. These films resemble those of Ru20Si14O65 and of TM-Si-N (TM=early transition metal) in the thermal stability of their amorphous structure, but Ir18Si15O67 films differ sharply from all others by their high chemical inertness against dry oxygen.