Efficient interfacial electrocatalysis requires rapid concerted proton−electron transfer (CPET) at the electrode surface, a process for which there is little mechanistic understanding. In aqueous media, multiple proton donors coexist, adding to the mechanistic complexity. Herein, we examine the rate of the hydrogen evolution reaction (HER) on Au, a proxy for CPET to Au, as a function of the concentration of exogenous phosphate and borate proton donors. We find that the reaction order in phosphate is 0.6, whereas the reaction order in borate is close to 0, indicating that phosphate, unlike borate, can outcompete water as a proton donor for interfacial CPET. Promotion in phosphate is substantial; the rate of the HER on Au in saturated potassium phosphate at pH 6.8 is identical to the rate of the HER on Au at pH 1. Additionally, we demonstrate that buffer-promoted CPET is a phenomenon that extends beyond Au to an Earth-abundant catalyst, NiS x . Kinetic data indicate that interfacial CPET cannot be viewed as a simple bimolecular reaction between the donor and a surface site, but that it first requires the formation of a preassociation complex embedded in the double layer. Our results emphasize that both the proton donor and the donor's environment control the rate of interfacial CPET and consequently have profound effects on the rate of heterogeneous electrocatalysis.