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The pH, one of the critical factors in determining the size distribution of gold nanoparticles (AuNPs), is not maintained at a constant value during the synthetic reaction. In this work, AuNPs were synthesized in the pH range of 5.5–10.5 and the pH was monitored in situ throughout the whole synthetic process. It was found that the citrate-based AuNP synthesis could be divided into two stages according to phases of pH change: a pH-decreasing early stage and a pH-increasing final stage. The narrowest size distribution at pH 7.5 can be well understood with these two phases of pH change.