The Izu-Bonin/Mariana convergent plate margin is characterized by a nonaccretionary forearc with numerous serpentinite seamounts that occur along a 50-km-wide ridge in the Izu-Bonin system and are distributed over a zone as wide as 90 km in the Mariana system. The seamounts are formed primarily by mud volcanism; however, seamounts in the two systems differ in tectonic setting and the materials that form them differ in compositional characteristics. These compositional characteristics suggest that there may be regional differences in the suprasubduction- zone mantle within the Izu-Bonin/Mariana system. In the Izu-Bonin forearc, the seamounts and the ridge on which they formed lie at the top of a wedge of low-velocity material that underlies the outer half of the forearc toe. This wedge is interpreted to be serpentinized forearc peridotite remobilized in response to subduction processes. In the Mariana forearc, seamounts form adjacent to major fault traces scattered over a broad zone in the outer half of the forearc. Studies of pore fluids from serpentinite seamounts show systematic variation in composition related to depth to the slab and to various devolatilization reactions that are controlled by pressure and temperature conditions at the decollement. The serpentinized peridotite materials erupted in mud volcanism indicate that the forearc mantle of the Izu- Bonin system has experienced a different history of both magmatic and melt-fluid interaction than those of the Mariana system. The mud flows at some of these seamounts bring up altered mafic rocks with compositional characteristics of both oceanic plate and island arc origin and fragments of metabasites that reflect the high-pressure and low-temperature conditions of the subduction zone. Some of these rock fragments are likely derived from the subducted slab. The three seamounts drilled during Ocean Drilling Program Legs 125 and 195 thus provide a window into suprasubduction-zone mantle processes, as well as those at work within the decollement itself.