A floating flux chamber, built of Plexiglas, has been developed to measure vapour-phase mercury fluxes over aquatic systems. The chamber is connected via Teflon tubes to a semi-automatic sampling device and to a mercury analyzer (battery-powered) to perform the determination of mercury emission in situ. Relatively low chamber blanks have been observed both in laboratory and in field conditions. The low detection limit of the analyzer (0.5 pg of mercury) allows measurements of mercury concentration in a short period of time (10 min). In this way it is possible to follow the real-time temporal trend of mercury emission. Some data obtained with this flux chamber in selected areas of the Mediterranean basin are reported.