Mercury(II)-modified carbon nanotubes can be readily prepared by reacting purified/oxidized carbon nanotubes (CNTs) with a Hg(NO3)2 aqueous solution. Two types of surface-confined Hg(II) species are formed and have been identified as (CNT-COO)2HgII and (CNT-O)2HgII. These two complexes have a surface concentration ratio of about 30%:70%, on the basis of data obtained from high-resolution XPS spectra, Raman spectroscopy, and electrochemical measurements. The electrochemical behavior of Hg(II)-modified CNTs adhered to electrode surfaces in contact with CH3CN (electrolyte) strongly depends on the nature of the working electrode used and the size of electrolyte cation. Significant voltammetric changes also are observed after the addition of water to the initially water-free acetonitrile electrolyte solution. At a glassy carbon electrode and using NaClO4 as the electrolyte, a proposed mechanism is operative. However, at a gold-coated quartz-crystal electrode, Hg formed after reduction reacts with the Au to form Hg-Au alloy which has a very positive stripping peak potential value compared to that for the Hg film formed on glassy carbon surfaces. The influence of the electrolyte cation size on the reduction of Hg(II)-modified CNTs is attributed to the intercalation of electrolyte cations.