Deep-sea hydrothermal vents represent a major source of heat and chemicals to the oceans and support endemic chemosynthetic biological communities. To fully understand the impact of hydrothermal activity upon the oceans, however, requires investigation of both the physical and the biogeochemical processes which are active in hydrothermal plumes and which serve to determine the net hydrothermal flux to the oceans. We have recently conducted a detailed multidisciplinary study of the lateral dispersion of the hydrothermal plume emitted from the Rainbow vent site near 36°15'N, Mid-Atlantic Ridge. Combining velocity measurements from a lowered ADCP, optical back scatter measurements from a deep-tow CTD and methane measurements from bottle samples we are able, for the first time in the Atlantic, to trace a neutrally buoyant plume for a distance of over 50 km. The path of the plume is seen to be heavily controlled by the local topography with a general northeast movement of water. Both particle and methane concentrations decrease downstream over the length of the observed plume. The dataset provides an excellent opportunity to study the mixing and biogeochemical processes active in a hydrothermal plume and estimate fluxes of biogeochemical constituents.