Although the electrochemical hydrogen evolution reaction (HER) on a Pt electrode is among the most studied electrocatalytic reactions, its reaction mechanism and exchange current density are still under debate. Particularly on the Pt catalyst, its facile reaction kinetics and lack of effective methods to compensate mass transport make it difficult to isolate kinetic and diffusion contributions. This study focuses on the quantitative description of mass transfer constraints arising from both H2 and phosphate buffer ions using a Pt electrocatalyst in near-neutral pH regions, which are overlooked in the literature despite the relevance to various (photo-) electrochemical reactions for water splitting and CO2 reduction. The established HER model that uses H+ as a reactant quantitatively breaks down the observed overpotentials with diffusion contributions of both hydrogen and buffer species while the intrinsic kinetics on Pt exhibits negligible contribution. The significance of electrolyte engineering, that is, the optimization of the electrolyte identity and molality, is confirmed to determine the overall HER performance. To maximize the HER performance on Pt at ambient temperature, the phosphate-buffer electrolyte should be adjusted to a pH close to pKa of the buffer that effectively minimizes the pH gradient and to the adequate molality of the buffer (typically ∼0.5 M phosphates) that H2 diffusion is maximized, which is related with H2 solubility and solution viscosity. At pH far from pKa, concentration overpotential of buffer ion dominates the overall performance, which requires highly dense buffer (>1.5 M phosphates) to minimize the pH gradient.