We demonstrate the synthesis of Ag nanoparticles (NPs) with diameters in the range 10-20 nm using two phytochemicals, gallic acid and protocatechuic acid, which have similar chemical structures but subtle differences in their chemical potentials. This subtlety in their potential difference improves the control over the reduction of Ag precursors such that their composition and concentration are optimized for both the nucleation and growth steps of NP synthesis. Both steps were carefully monitored using UV -vis spectrophotometry and electron microscopy. The surface interaction energy of the phytochemicals on the NPs was characterized using computational simulation, and electrochemical analysis was performed to calculate the chemical reducibility of the phytochemicals to Ag+ ions by cyclic voltammetry. A plausible mechanism of size control was suggested. This novel and valuable method to control the size and the size distribution of Ag NPs enables new applications in materials science, biomedical science, and chemical sensor development. (C) 2016 Elsevier B.V. All rights reserved.