We experimentally investigated the trace element compositions of fluids released during breakdown of subducted serpentinites. Serpentinites contain significant amounts of fluid-mobile elements such as B, Cs, As, and Ba, which during seafloor alteration are incorporated into mantle rocks. During the later high-pressure breakdown of the serpentinites, these trace elements are redistributed among the residual olivine, orthopyroxene, minor chlorite, and fluid. We find that B is far more compatible in these minerals than previously assumed; it has a fluid/residue partition coefficient (F/RD) of 3–5. Most other fluid-mobile elements (Cs, As, Ba, Pb) are strongly enriched in the fluid and exhibit expected F/RD values of 30–250. Serpentinites are possibly the most important sink for B in subduction zones; the experimental results suggest that significant B is recycled into the deep mantle. Furthermore, high B concentrations in mantle olivines might be a fingerprint for previous metasomatism or serpentinization.