Moho depth and Vp/Vs estimates from stacking phases of receiver functions along the Aleutian island arc give new constraints on its composition and structure. They expand on the current understanding of island arcs and their relationship to continental crust production. We also present an approach for including constraints from active-source data in receiver function analysis in a region with sparse data coverage to complement this analysis. Moho depth averages 37.5 km with an average uncertainty of 2.5 km along the entire arc. Excluding the westernmost island of Attu yields an average crustal thickness of 38.5 ± 2.9 km. The Vp/Vs ratio decreases moving eastward along the arc with an average value of 1.80 in the western and central portion of the arc built on oceanic crust, but 1.63 in the eastern section built on continental crust. This may reflect tectonic and compositional changes along the arc. However, overall the arc appears more mafic than continental crust. Near-constant crustal thickness, despite significant compositional changes, may indicate that nonmagmatic processes such as erosion and isostasy act to regulate arc thickness. Additionally, strong conversions from an upper crustal magma chamber are observed beneath Akutan Island, confirming and clarifying the geometry of the magma body inferred from other techniques. They indicate a volcanic body much larger than the eruptive edifice, a feature that must persist between eruptive cycles.