Public Abstract: Carbon-supported platinum–rare earth nanoalloys are promising electrocatalysts for the oxygen reduction reaction. However, their structure–activity–stability trends are poorly understood. Herein, we followed the evolution of Pt–Nd/C nanoalloys during the electrochemical surface conditioning, i.e., prior to the initial evaluation of electrocatalytic activity, and observed that their compositional, morphological, and structural ex situ properties are considerably modified by the electrochemical activation step. It is these stabilized properties, therefore, that should be considered when discussing the electrocatalyst beginning-of-life state for the structure–activity–stability relationships rather than those determined ex situ on the as-synthesized electrocatalyst.