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Abstract An improved understanding of catalyst dynamics for the oxygen evolution reaction (OER) in acid is critical for informing the development of highly efficient, stable, and cost‐effective OER catalysts for proton exchange membrane water electrolysis applications. Herein highly tunable, active, and dynamic Ir 5+ materials are studied, Ln₃IrO₇(Ln = Pr, Nd, Sm, and Eu). Leveraging a combination of in situ and ex situ characterization, as well as an advanced mercury underpotential deposition technique for Ir surface site quantification, the dynamic nature of Ln₃IrO₇materials throughout electrochemical activation under OER conditions is characterized. The trends are elucidated between intrinsic OER activity, surface Ir site quantity, and metal site dissolution behavior as tuned by the Ln site's atomic number. A critical relationship is uncovered to show that maintenance of excellent OER activity throughout performance testing is correlated with a catalysts’ ability to preserve a high degree of Ir enrichment, where heightened stability of Ir sites interestingly parallels reduced stability of Ln sites throughout testing. It is found that as the Ln site's atomic number is decreased, the materials’ intrinsic OER performance improves, due to an increased thermodynamic driving force for Ln dissolution, which is hypothesized to enable the maintenance of highly active Ir‐based surface motifs.