Conical and Torishima Seamounts in the Mariana forearc are serpentinite protrusions onto the seafloor that result from large scale hydration of the forearc mantle wedge by dewatering of the underlying subducting slab. We have analyzed the stable isotope compositions and sulfur contents of serpentinized peridotite sampled by ODP drilling of these seamounts to investigate the transport of sulfur from the subducting slab to the mantle wedge. Low-chlorinity fluids actively venting from the summit of Conical Seamount are derived from dehydration reactions in the subducting slab, 30 km below. Twenty three variably serpentinized harzburgites (to 295 m below seafloor) contain 1-105 ppm sulfide-S having delta S values of -6.7 to +9.8 per mil, and 20-540 ppm sulfate-S having delta S values of -1.1 to +21.8 per mil. Four highly serpentinized peridotites from inactive Torishima Seamount have sulfide-S contents generally too low for isotope analysis (<20 ppm), but one sample has delta S (sub sulfide) = 4.7 per mil. Sulfate-S contents are variable (15-3590 ppm) and delta S (sub sulfate) is low (-20 to +6.3 per mil). Six serpentinites from Conical Seamount have delta D values of -29.5 to -84.0 per mil. Coarser grained samples have the highest values and probably preserve metamorphic values, whereas fine grained samples may have exchanged hydrogen with seawater at low temperatures. Oxygen isotope analyses are underway in order to further constrain temperatures and fluid compositions. The serpentinized peridotites record a complex history of fluids and serpentinization at depth and during exposure at the seafloor. Elevated delta S values of sulfide in serpentinite (up to 9.8 per mil) compared to mantle sulfur (0 per mil) are the result of input of S-enriched sulfur from sediments in the subducting slab, consistent with results for arc lavas. Locally low delta S values of sulfide (-1.1 to -5.5 per mil) in serpentinized peridotite probably result from microbial sulfate reduction within the seamount edifices, consistent with results for interstitial waters in serpentinite muds on Torishima seamount. The delta S of sulfate in the serpentinized peridotites ranges from seawater values near the surface of Conical Seamount (21.8 per mil) to low values predominatly reflecting oxidation of sulfide on the seafloor and during sample storage and chemical extractions.