<p>The microenvironment at electrochemical interfaces plays a crucial role in governing electrode-mediated electron transfer processes. However, elucidating the complex effects of the microenvironment remains challenging. The Butler–Volmer equation has been used in deducing reaction mechanisms and identifying rate-determining steps, but its empirical nature makes it challenging to deduce the molecular-level picture of interfacial electron transfer processes. By contrast, the application of the Marcus–Hush–Chidsey (MHC) electron transfer theory has been constrained by its tenuous connection to experimentally measurable parameters beyond reaction rates. Here we develop a mechanistic framework based on the MHC theory to systematically analyse the cation effect on the Au-catalysed CO₂ reduction reaction using experimentally accessible variables. Our analysis reveals consistent trends for both inorganic and organic cations through thermodynamic and kinetic parameters derived from the MHC theory, with potential applications for probing ionomer–electrode interface microenvironments. This study establishes a universal strategy for investigating interfacial microenvironments in electron transfer processes by bridging theoretical parameters with experimental descriptors.</p><p></p>