Questions of electrocatalytic selectivity remain unanswered for many technologically relevant processes in heterogeneous electrocatalysis. A characteristic example is the oxygen reduction reaction (ORR) which can proceed either through a four-electron (4e -) reduction to form H 2 O, or through a two-electron (2e -) reduction to form the partially-converted H 2 O 2 product. Both of these processes hold great practical value in their own right: the 4e -ORR is key to optimizing fuel cell efficiency, while the 2e -ORR provides a sustainable route to producing H 2 O 2 which is an important industrial chemical for e.g. water purification, pulp and paper manufacturing, etc. And yet, the origin of ORR selectivity between these two products is still not properly understood. Experiments clearly show that certain catalysts can produce H 2 O 2 at low potentials, even almost exclusively, with close to 100% selectivity, cf. e.g., refs 4-7. This preference is empirically observed for weak-binding catalysts, such as carbon-based materials or noble metals like Ag and Au (depending strongly also upon the facet orientation and reaction conditions). Many have tried to explain these effects through e.g. suppressed O-O bond scission due to surface site blocking by electrolyte ions, or modified adsorbate binding in the presence of co-adsorbed OH or H 2 O species. Regardless of the nature of the active site considered, however, the theoretical limiting potential analysis will always favor breaking the O-O bond to form H 2 O which is by far the most thermodynamically stable product. The discrepancy between experiment and theory in predicting ORR selectivity is illustrated in Fig. for the Au(111) catalyst surface. The plotted free energy reaction diagrams outline modeled paths for the 4e -ORR (in blue) and 2e -ORR (in red). The thermodynamic energy levels of reaction intermediates are computed from DFT following common practice (cf. Methods) and shown here for standard potential conditions (0 V vs. the reversible hydrogen electrode, RHE). Comparing the two ORR paths clearly shows the thermodynamic preference for breaking the O-O bond already in the second PCET step: leaving an O adatom at the surface with the concomitant release of a first H 2 O molecule is significantly more stable than the reductive desorption to H 2 O 2 . This result persists when shifting the potential closer to operating conditions and is in stark contrast to experiments which measure only selective H 2 O 2 formation over Au(111). Resolving this discrepancy will require going beyond the simple picture of PCET thermodynamics shown in Fig. . This proposition is far from new. It has, however, remained largely unexplored due to the practical difficulty of modeling the electrified metal/water interface and predicting reliable electrochemical barriers. In this study, we investigate the kinetic competition that drives ORR selectivity on Au(111). Our approach uses large-scale ab initio molecular dynamics (AIMD) along with umbrella sampling to obtain free energy profiles for key, competing steps in the ORR mechanism. This comparison includes not only PCETs, but also so-called "chemical" steps with no (H + + e -) transfer such as desorption or surface dissociation. While AIMD provides a full atomistic account of the dynamic H 2 O solvent, we explicitly charge the interface in order to explore favorable reaction paths under more realistic ORR potential conditions.