Chlorine isotope ratios were determined for volcanic gas, geothermal well, ash, and lava samples along the Izu-Bonin-Mariana volcanic front, serpentinite clasts and muds from serpentine seamounts (Conical, South Chamorro, Torishima), basalts from the Guguan cross-chain, and sediments from Ocean Drilling Program (ODP) Sites 800, 801, 802, and 1149. There is no systematic variation in delta (super 37) Cl values along the volcanic front in either gas or ash samples. In contrast, distinct variations occur across the arc, implying variations in the fluid source at different depths within the subduction zone. Serpentinite clasts and serpentine muds from the seamounts tap a source of approximately 30 km depth and have delta (super 37) Cl values of structurally bound chloride of +0.4 per mil + or - 0.4 per mil (n = 24), identical to most seafloor serpentinites, suggesting a serpentinite (chrysotile and/or lizardite to antigorite transition) fluid source. Tapping deeper levels of the subduction zone (approximately 115-130 km depth), volcanic gases and ashes have delta (super 37) Cl values averaging -1.1 per mil + or - 1.0 per mil (n = 29), precisely overlapping the range measured in sediments from ODP cores (-1.1 per mil + or - +0.7 per mil, n = 11) and limited altered oceanic crust (AOC). Both sediments and AOC are possible Cl sources in the volcanic front. The Guguan cross-chain basalts come from the greatest depths and have an average delta (super 37) Cl value of +0.2 per mil + or - 0.2 per mil (n = 3), suggesting a second serpentine-derived source, in this case from antigorite breakdown at approximately 200 km depth.