We demonstrate for the first time the use of Au nanowires for the electrical detection of elemental and ionic mercury. By monitoring changes in resistance upon exposure to Hg vapor, concentrations as low as 6.25 mu g/m(3) (2 X 10(-10) M or 5 ppb) were detected. Similarly, we used HgCl2 in an aqueous solution as a source for Hg ions and were able to detect Hg2+ concentrations as low as 10(-8) M. We also demonstrate the impact of the sensor surface to volume ratio, substantiate the model of a surface origin for the sensitivity, and underscore the advantages of the Au nanowire structure over the thin-film structure for Hg sensing. (c) 2006 The Electrochemical Society.