At least two different slab-derived components have been proposed to be present in arc volcanoes: a sediment component and a component derived from altered mafic oceanic crust. Despite a widely held belief of the involvement of these components in the genesis of arc volcanoes [1,2] the unambiguous identification of these components and discrimination of their chemical signatures from residual phase mineralogy remains difficult. We propose stable molybdenum isotopes as a novel tracer for subduction components in arc magmas. Molybdenum has seven stable isotopes, which have been shown to fractionate during the incorporation of dissolved Mo into oceanic sediments. Under oxic conditions, Mo slowly adsorbs to particles in the sediment, a process that is particularly efficient when Fe-Mn oxides are present. The adsorption of Mo is associated with isotopic fractionations from delta (super 97/95) Mo (sub seawater) nearly equal 1.8 ppm to delta (super 97/95) Mo (sub oxic-sediments) < 0 ppm. Under anoxic conditions, Mo is quantitatively removed from the water column and sediments with a heavy isotopic composition are produced [3,4]. We measured Mo isotopes on basalts from the Mariana arc as well as representative samples of subducting sediments from ODP sites 800, 801 and 802 using a double-spike technique [5]. The sediments are light in their isotopic composition with delta (super 97/95) Mo (sub sediments) < 0 ppm, consistent with the incorporation of Mo into the sediment under oxic conditions. The arc basalts are enriched in Mo relative to Pr, an element with similar degree of incompatibility during mantle melting. The Mo isotopes in the arc basalts correlate well with Mo/Pr, Ba/La and Ce/Pb ratios, with samples containing a larger sediment component [6] also having the lowest delta (super 97/95) Mo. Less incompatible element enriched samples are isotopically heavy and their delta (super 97/95) Mo exceeds the range of ocean island basalts and continental material. They presumably trace a fluid derived from deeper parts of the subducted lithosphere. [1] Plank & Langmuir (1998) Chem. Geol. 145, 325-394. [2] Plank (2004) J.Petrology, 5, 921-944. [3] Anbar & Rouxel (2007), Ann. Rev. Earth Planet. Sci., 35, 717-746. [4] Siebert et al. (2003) EPSL 211, 159-171. [5] Archer et al. (2008) Nature Geosc., 1, 597-600. [6] Elliott et al. (1997) JGR, 102, 14,991-15,019.