Monitoring the reflectivity of optically thin Au films on the ends of multimode optical fibers during exposure to Hg vapor provides a new means of determining the nature and extent of chemisorption and reaction of Hg with Au. Upon Hg vapor exposure, a freshly deposited Au film shows an initial increase in reflectivity, followed after some time by a substantial reflectivity decrease. The initial reflectivity increase is a result of the chemisorption of Hg on the Au, augmenting the film’s optical thickness. The subsequent reflectivity decrease, which is observed only for saturated or near‐saturated Hg vapor concentrations (p/psat>0.1), is a consequence of amalgamation of the Au by the Hg. The amalgamation process, which begins at grain boundaries when multilayers of Hg exist on the surface of the Au, results in the formation of large voids in the Au film. Energy‐dispersive x‐ray fluorescence spectroscopy shows the resulting amalgam to have the approximate composition Au2Hg3. Reflectivity measurements have been made as functions of Hg partial pressure and of Au film thickness; models are presented which describe both behaviors. Detection of Hg vapor in the parts per 109concentration range has been demonstrated, suggesting that this effect could be used to monitor Hg vapor concentrations at the Occupational Safety and Health Administration exposure limits.