Electrocatalytic CO2 reduction to liquid fuels and chemical feedstock, powered by renewable electricity, is an important approach for storing renewable energy and closing carbon cycle. Here, we study the pH effect on CO2 electroreduction selectivity over size-selected Au nanoparticles (NPs) in citrate buffer solutions with pH 3.7-6.0. A maximum CO Faradaic efficiency of 96.3% is achieved over Au NPs at pH 6.0, and synthesis gas with tunable H-2/CO ratios (0.04-22) can be produced in the studied pH range. By careful kinetic analysis, the observed pH dependence reveals a most likely mixed mechanism of concerted proton-electron transfer and sequential electron-proton transfer in the initial step of CO2 activation. The pH-dependent selectivity changes over Au NPs can be rationalized by the difference in the pH dependence of CO2 electroreduction and concurrent hydrogen evolution reaction. This work provides new guidelines into controlling activity and selectivity of CO2 electroreduction via efficiently tuning electrolyte pH. [GRAPHICS] Selectivity of CO2 electroreduction over size-selected Au nanoparticles is tuned via electrolyte pH.