Measurements of the temperature and composition of effluent from vents on the sea floor can be used to deduce the in-situ density of this fluid, which is required for calculations of flow in the chimneys and through their porous walls. This density is, however, not directly relevant when calculating the buoyancy flux in the plume above a smoker. It is the asymptotic buoyancy flux, following extensive dilution with seawater, which is required when estimating the height of rise of plumes in a stably stratified ocean, and when calculating the criterion for reversal of buoyancy due to non-linear mixing effects. The results of mixing calculations show that the effluent from hydrothermal vents on the sea floor will exhibit reversing buoyancy if the ejected fluid has a temperature of 300°C and a salinity greater than ∼ 8 wt.% NaCl. If the temperature of the effluent is 200°C the salinity required for reversing buoyancy falls to ∼ 5.5 wt.% NaCl. Measurements of temperature and salinities of sea-floor hydrothermal fluid suggest that fluids with the characteristics required to form reversing plumes are ejected at the sea floor. The possibility that reversing plumes may be found has important implications for the formation of massive sulfide deposits.