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. (C) 2015 Elsevier B.V. All rights reserved.