The thermal oxidation of single-crystalline Ir(100) films toward rutile IrO2(110) has been investigated in situ. Initially, a carbon-free surface covered with a complete monolayer of oxygen forms, followed by a rather long induction period with nearly constant oxygen coverage during which the work function of the surface changes continuously. Subsequently, IrO2 rutile with a thickness above 3 nm grows suddenly, providing direct evidence that the oxide film is formed via an autocatalytic growth mechanism.