Iridium oxide is still considered the superior material for the oxygen evolution reaction (OER) in acidic environment, as it maintains both high activity and stability, making it the benchmark material for green hydrogen production. Despite the numerous studies describing the activity of IrO2 materials for the OER, it is crucial to have a thorough understanding of its functional principles in order to optimize and rationalize its utilization. Towards this objective, we have been able to produce porous iridium-based mixed oxides using aerosol chemistry via a sol-gel process, , to decouple the electronic processes from the structural transformations. Indeed, the oxidation of iridium from IrO2 occurs upon calcination, but is not related to the crystallization of the material itself. The measurements demonstrate that short-range ordering, corresponding to sub-2nm crystal size, is controlling the activity of the materials. This is the case regardless of the initial oxidation state and composition of the calcined iridium oxides. Regarding other types of porous materials, we have also been able to prepare hierarchical structures of IrO2 based on Pearson’s hard and soft acid-base theory and characterize their outstanding activity toward the OER. [1] Chatenet, M. et al., Chem. Soc. Rev. 2022,51, 4583-4762. [2] Faustini, M. et al., Adv. Energy Mater. 2019, 9, 1802136. [3] Duran, S. et al., ChemElectroChem 2021, 8, 3519-3524. [4] Elmaalouf, M. et al., Nat. Commun. 2021, 12, 3935. [5] Elmaalouf, M. et al., Chem. Sci. 2022, 13, 11807-11816.