<p>Photoelectrochemical water oxidation to generate H₂O₂ is a clean and promising method. Its performance is strongly dependent on electrolyte species, in which the Faradaic efficiency is considerably promoted by HCO₃⁻ anion. The kinetic mechanism is under debate, which is highly desired but challenging. Herein, we reveal the charge dynamics and reaction kinetics in the H₂O₂ evolution from photoelectrochemical water oxidation by time-resolved spectroscopic techniques. The H₂O₂ evolution reaction exhibits the same first-hole transfer dynamics as that in O₂ evolution reaction. The rate law analysis indicates that H₂O₂ evolution reaction exhibits the first-order reaction kinetics, demonstrating that the rate-determining step in 2e⁻ water oxidation reaction for H₂O₂ evolution is the consumption of the first-hole intermediates. Importantly, the HCO₃⁻ anion accelerates the consumption of the first-hole intermediates in 2e⁻ water oxidation reaction by about 30 fold in rate constants or 60 fold in turnover frequency relative that in 4e⁻ water oxidation reaction. This work sheds light on the control strategy for reaction selectivity by modulation of reaction kinetics in catalysis.</p>