Preparation of size-controlled gold nanoparticles is required for their use in applications. Electroless growth of gold nanoparticles using Au seeds with mean diameter of 12.5 nm and hydroquinone was performed and the growth process was analyzed. The pH dependencies of the redox potentials of hydroquinone and Au(III) ionic species were evaluated to investigate the growth mechanism of the nanoparticles in detail. Furthermore, the deposition behavior of Au in the growth solution was clarified by measuring the deposition weight of Au and the mixed potential during the growth of Au nanoparticles. Size control of Au nanoparticles was achieved by changing the concentration of Au seeds in the Au growth solution, allowing Au nanoparticles with a mean diameter of 38.7-63.5 nm to be successfully obtained. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.065207jes] All rights reserved.