Both 10Be and 9Be concentrations have been measured in a hydrothermal fluid from the sediment-starved East Pacific Rise at 21°N as well as in fluids and associated sediments from the sediment covered Guaymas Basin and Escanaba Trough hydrothermal systems. The results show that while the 9Be concentrations in fluids from both types of systems are comparable, about three orders of magnitude higher than that of the seawater supplying the systems, their 10Be concentrations are significantly different. The 10Be concentration in the solution emanating from the studied sediment-starved hydrothermal system is at least two times lower than that of the Pacific bottom water. By contrast, the 10Be concentration in hydrothermal solutions from sediment-hosted environments are enriched over ambient seawater and are at least one order of magnitude higher than that measured in the 21°N fluid. The sedimentary cover being the only available source of 10Be in these systems, these data demonstrate that 10Be is an unambiguous tracer of interactions between hydrothermal fluids and recent sediments. As an example, this study examines hydrothermal solutions from the sediment-starved Mariana Trough back-arc basin which have compositions similar in many respects to those from sediment-hosted environments. However, these fluids are depleted in 10Be relative to ambient seawater, which precludes interaction with recent sediment as the cause of their peculiar chemical composition.