The influence of the deposition temperature during the reactive sputtering process on the microstructure of thin Ir and IrO ₂ films deposited on oxidized Si substrates was investigated and related to the oxygen barrier effectiveness. For this purpose differential thermal analysis combined with residual gas analysis by mass spectrometry was used for the investigation of the microstructural and chemical behavior of the as-sputtered IrO ₂ films upon heating. Moreover, in situ stress relaxation analyses up to 900°C, in and ex situ x-ray diffraction measurements were done for various annealing conditions. The investigated polycrystalline IrO ₂ films exhibited a large compressive stress and a distorted lattice due to the sputter deposition process. It is demonstrated that a high deposition temperature involves a delayed relaxation of the IrO ₂ grains which is causing an extrinsic, enhanced defect controlled oxygen mobility for the annealing temperatures below the recrystallization. The well-known low intrinsic oxygen diffusivity was only found in those samples which show - in addition to the recovery process - a recrystallization at low temperatures and thus a formation and growth of a new generation of grains with a lattice spacing as in bulk IrO ₂. Moreover, the oxygen diffusion in Ir films was investigated and the oxygen was found to penetrate the Ir films very quickly at elevated temperatures. The microstructure of the films was investigated by cross sectional transmission electron microscopy and it is shown that the cold-sputtered columnar IrO ₂ films protect the underlying layers from oxidation during a 700°C high temperature oxygen anneal with an optimized Ir/IrO ₂ oxygen barrier stack.