We developed a microkine�c model of the system (Figure and Suppor�ng Informa�on) to replicate the observa�ons from GDE ICP-MS prior to AST at 20 o C in 0.1 M HClO 4 and evidence our hypothesis on the pivotal role of local pH. The model assumed the ini�al proton concentra�on and poten�al in the catalyst layer is homogeneous. We focus on the Fe concentra�on observed in GDE ICP-MS at 0.75 V RHE, iR-free and 20 °C, corresponding to a current density of -15 and 0 mA cm -2 geo under O 2 and Ar supply, respec�vely. The void volume (ε) in the catalyst layer was adjusted to semi-quan�ta�vely simulate the �me evolu�on of the Fe concentra�on signal monitored by GDE ICP-MS in Ar-saturated electrolyte (Figure and). The value of the proton consump�on rate constant (k r ) and ε were then varied to replicate the Fe concentra�on signal measured in O 2 -saturated electrolyte (Figure ). Good agreement between experiment and simula�on are reached for the range of values considered (0.2≤ ε ≤0.4 and 100≤ k r ≤400 s -1 ). Addi�onally, values for the tortuosity factor, τ (=1/√ε) were within previously reported ranges (1.8 ≤ τ ≤2.2). Figure displays the corresponding simulated pH profile in the catalyst layer. The simula�ons predict a significantly lower concentra�on of detected Fe ca�ons during O 2 reduc�on (Figure ). This phenomenon is atributed to the precipita�on of Fe 3+ ca�ons under the local condi�ons in the catalyst layer, with the Fe concentra�on resul�ng from the balance between Fe precipita�on and redissolu�on. Indeed, simula�ons indicate that at -15 mA cm -2 geo , the local pH at the interface between the Fe-N-C layer and the electrolyte solu�on is approximately 1.5 (Figure ). There is then a substan�al and rapid increase in pH moving into the bulk catalyst layer (far from the liquid electrolyte), reaching pH values ca. 1. The fate of Fe (and other metal species) can vary through the catalyst layer and should be considered when conduc�ng operando and post-mortem studies.