We have studied the adsorption of Hg atoms onto gold and silver nanoparticles by optical spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis. Mercury adsorption induces a blue shift of the surface plasmon mode of gold and silver nanoparticles. Two trends are observed in the UV-vis spectra. First, silver particles experience a larger blue shift than gold particles (given the same particle size and Hg surface coverage). Second, smaller Au and Ag particles experience a greater blue shift than larger particles. We have calculated the UV-vis absorption spectra using a core/Hg(shell) model and found good agreement between the experimental results and theoretical calculations. The results suggest the basis for a novel colorimetric assay for Hg.