A glassy carbon electrode (GCE) modified with gold nanoparticles (GNPs) was used for the determination of sub-nanmolar amounts of mercury in water samples using differential pulse stripping voltammetry (DPSV) and electrochemical impedance spectroscopy (EIS). The GNPs/ GCE electrode revealed a significant improvement for mercury detection compared with the unmodified GCE. Under the optimized experimental conditions, the modified GNPs/GCE revealed broad linear ranges of 1 to 500 nM (LOD = 0.1 nM) and 10-50 pM (LOD = 0.05 pM) for DPSV and EIS, respectively. The limits of detections obtained are much below the corresponding guideline value from the World Health Organization (WHO): The proposed DPSV method was successfully applied to the analysis of mercury content of tap water samples. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.011304jes] All rights reserved.