Understanding the structure evolution, kinetics, and mass transfer for the oxygen reduction reaction (ORR) at the ionomer-catalyst interface is fundamental for the development of anion exchange membrane fuel cells (AEMFCs). Herein, we investigate the structural evolution of ionomer-Pt interfaces during the activation process of polycrystalline Pt (poly-Pt) electrodes and their ORR kinetics and mass transfer characteristics at steady state. The results suggest the ionomer thickness as a critical factor in determining the Pt surface structure and the flux of the O 2 diffusion, which in turn affect the subsequent kinetic and mass transfer of the ORR on ionomer-Pt electrode interfaces. Thicker ionomer film leads to a more severe evolution of electrochemical features during the activation process, likely caused by forming more less-active Pt clusters at the ionomer-Pt interface. Thus, the ORR kinetic activity at the steady state decreases with the increase in ionomer thickness. Concurrently, the thicker ionomer leads to a reduced diffusion flux of O 2 , culminating in a lower limiting current density for the ORR. Additionally, we calculated the diffusion coefficient and solubility of O 2 within the FAA-3 alkaline ionomer film, with a comparative assessment against those in the proton exchange membrane (PEM). These findings offer valuable insights into the ionomer-Pt interface in AEMFCs and their effects on performance.