The degree of transportation of beryllium with H2O is measured under the upper mantle P/T conditions in order to examine the incorporation of 10Be in subduction zone magmas. 5.4¿0.3% of original beryllium in the starting serpentinite is extracted with H2O from the system through the dehydration reaction of serpentine at 850¿ C and 1.5 GPa. It follows that the partial melting of oceanic lithosphere including 10Be-rich sediment layer is not necessarily required for inclusion of the isotope in island-arc lavas. We suggest that the dehydration processes in the subducted slab and mantle wedge materials dragged downward by the slab subduction govern the concentration of 10Be in arc lavas. ¿ American Geophysical Union 1988