The chemical composition of recent marine sediments and pore waters in areas of shallow underwater volcanic vents in the Bay of Plenty, New Zealand, was studied. Values of pH in the sediments in active sites were higher than in control sites, and concentrations of Fe, Cu, and Ni were elevated; however, the general chemical composition of the sediments and pore waters was rather similar to that of common shelf sediments. The chemical composition of pore waters agrees well with the usual diagenesis of these sediments; the increase in temperature might influence the rates of diagenetic processes. No signs of uniform seeping of volcanic waters through the sediments were detected. Hydrothermal waters and gases are preferentially transported through sulfur tubes. These tubes and sulfide concretions replacing magnetite were collected at and near the hydrothermal vents.