The instability of iridium oxide at high temperature has long been a bottleneck for in growing pyrochlore iridate thin films in a vacuum chamber. To overcome this problem, we investigated the chemical instability of IrO2 thin films, which are the simplest form of iridate, via in-operando spectroscopic ellipsometry (SE). We observed that IrO2 thin films undergo IrO2 dissociation and IrO3 gas formation depending on the thermodynamic conditions. The chemical kinetics observations of IrO2 were confirmed by ex-situ X-ray diffraction and atomic force microscopy. SE experimental data were compared with models used to describe the evolution of the two chemical reactions. Real-time in-operando SE analysis based on the Maxwell Garnett theory yielded a precise IrO2 dissociation speed for the given thermodynamic conditions. Moreover, the real-time in-operando SE technique allowed us to observe the phase transition from solid IrO2 to gaseous IrO3. This study on the chemical instability of IrO2 at high temperature affords insights into a new method for in-situ pyrochlore iridate and other iridates thin-film growth.