We describe and experimentally verify the mechanism of area-selective electroless deposition of gold nanostructures at interfaces between locally induced naked silicon surfaces and pure HAuCl4 solution. We previously demonstrated that gold nanostructures selectively grow on focused ion beam (FIB)-irradiated areas of a silicon substrate when the substrate is exposed to pure HAuCl4 solution. We revealed that silicon dangling bonds formed by FIB irradiation reduce Au(III) ions and initiate the nucleation of gold. Here, we focus on the growth stage. Quantitative estimate of the amount of gold grown reveals that silicon dangling bonds explain only a fraction of the total amount of gold. Thus, the growth mechanism is different from the nucleation mechanism. In the growth stage, soon after silicon/gold interfaces form via gold nucleation, electrons in the crystalline silicon near the interfaces reduce Au(III) ions through the interfaces and the growing gold. This mechanism is consistent with the Fermi level (chemical potential of electrons), which decreases in the order silicon > gold > HAuCl4 solution in the isolated form. Double exposure to the same and different HAuCl4 solutions shows the results that support the mechanism, and allows further control of gold morphology. (C) 2016 The Electrochemical Society. All rights reserved.