Nanoporous gold (NP-Au) is used in various energy applications. However, little attention has been paid to the nature of surface films which can grow via side reactions during the formation of NP-Au by selective leaching. These films could significantly impact both the kinetics of NP-Au formation by selective leaching, and the performance of this NP-Au in specific energy applications. While it has been reported that complete removal of Ag from a Au–Ag parent alloy by selective leaching is kinetically more sluggish in HClO₄ than in HNO₃, and that the resulting structure size of NP-Au is smaller when formed in HClO₄ compared to HNO₃, these findings were not attributed to the growth of surface film via side reactions during selective leaching. In this work, small-angle and wide-angle X-ray scattering (SAXS and WAXS) are used in real time during NP-Au formation by selective electrolytic leaching of Ag from a Au–Ag parent alloy in 1 M HClO₄ and 1 M HNO₃ to demonstrate that the sluggish dealloying behavior in HClO₄ results from the growth of a surface film during dealloying. In HClO₄, Ag⁺ ions released from the Au–Ag working electrode spontaneously react with Cl⁻ ions released from ClO₄⁻ reduction at the counter electrode to form an insoluble AgCl passive film on NP-Au. The AgCl hinders Ag dissolution and surface diffusion of Au atoms, slowing down the reaction kinetics and the rate of ligament growth. Our findings shed light on dealloying reaction mechanisms and will contribute to better control of the morphology and surface chemical state of nanoporous metals for energy applications.