Iron incorporation is essential for the record activity of NiFe-(oxy)hydroxides to oxygen evolution reaction (OER), but the details of how Fe affects catalysis remain under active investigation. In this work, we present a double thin-layer strategy for finding unique and solid evidence for the role of Fe in the OER mechanism. A thin-layer catalyst of a few nanometers of thickness was deposited on a Ni substrate and a thin-layer electrolyte of 0.1 mm thickness was created using a thin-layer spectroelectrochemical cell. The OER activity, the catalyst composition, and the electrolyte species were investigated together as a function of the Fe deposition time. The results show that trace Fe incorporation favors the formation of β-NiOOH in the thin-layer catalyst and effectively suppresses the dissolution of NiOOH into the electrolyte. The results of double-potential step chronoabsorptometry and cyclic voltabsorptometry demonstrate the potential-dependent formation of a Ni3+ intermediate in the electrolyte and, more importantly, the dissolution suppression effect due to Fe incorporation. These findings link the role of Fe in OER catalysis to the increased insolubility of Ni3+ active sites and highlight the importance of paying close attention to the active-site stability of an electrocatalyst impaired by the electrolyte at a reaction potential.