Water carried into subduction zones with the down-going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge, triggers partial melting and affects subduction zone seismicity. Partially serpentinized peridotite may be a significant reservoir for water in the subducted plate, the mantle wedge and the overriding plate. Here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic P-wave velocities. In partially-serpentinized ultramafic rocks, a 1% decrease in P-wave velocity corresponds to a 2.4% increase in serpentine content, and a 0.3% (0.18 moles/m3) increase in H2O content (up to a maximum of 13%). Where there is evidence of serpentinization, mantle serpentine content is typically ~15%, corresponding to 4--5 wt% H2O (6--10 moles/m3).