Profiles of both 10Be and 9Be concentrations have been measured in sediments from the DSDP, Leg 64, Site 477 core. Established in the deep Guaymas Basin, in an area of high heat flow, this site offers the opportunity of studying hydrothermal alteration induced by two distinct hydrothermal systems, those caused (i) by the intrusion of hot sills into porous sediments and (ii) by heat associated with a magma chamber. The results show that the distribution of both beryllium isotopes between two operationally defined sedimentary phases (material susceptible to hydroxylamine leaching and residual material) changes with the degree of hydrothermal alteration. A decrease in 10Be 9Be in the leachable phase of altered sediments relative to unaltered sediments indicates that hydrothermal alteration is not limited to a simple chemical leaching but involves redistribution of the beryllium isotopes between metamorphosed sedimentary phases. In addition, the similarity of the mean 10Be 9Be ratios associated with the leachable and the residual phases in the part of the core which has experienced greenschist facies metamorphism suggests that this process leads in the prevailing geophysical conditions to isotopic homogenization. A close relationship exists between 10Be and MgO concentrations in altered sediments from this core. This supports earlier observations that beryllium is not easily removed from the sedimentary column and indicates that this remains the case even under the conditions encountered in submarine hydrothermal systems.