Indium oxide thin films were grown onto Si and quartz substrates by d.c. reactive sputtering of elemental indium. X-ray diffraction and transmission electron microscopy studies confirmed the single-phase and polycrystalline nature of the films. Secondary ion mass spectrometry investigations of In 2 O 3 /Si structures showed the formation of an inhomogeneous interface region ∼20 nm thick between In 2 O 3 and Si. The overall feature of the interface remained the same under annealing in an oxygen atmosphere, but annealing in an argon atmosphere drastically altered the nature of the interface. The observations indicate that interface formation and stability depend critically on the availability of oxygen.