We present the results of an experimental study of the partitioning of water between common upper mantle minerals (olivine, orthopyroxene, clinopyroxene, garnet) and silicate melt, consisting of 352 measurements in 23 hydrous melting experiments conducted over a broad range of melt water contents (1.7–25 wt.% H₂O) at upper mantle conditions (1–4 GPa; 1000–1380 °C). Our data show that, at water-undersaturated conditions, incorporation of H₂O into olivine and pyroxenes is accomplished through two independent mechanisms: a coupled substitution in which H⁺ and Al³⁺ replace Si⁴⁺ in the mineral structure, and the substitution of 2H⁺ for Mg²⁺ previously identified in minerals hydrated at water-saturated conditions. At upper mantle temperatures and pressures < 2 GPa, these two substitution mechanisms appear to contribute approximately equal amounts of water to olivine; at higher pressures, the fugacity-dependent 2H⁺–Mg²⁺ substitution dominates. For orthopyroxene, coupled substitution of H⁺ and tetrahedral Al³⁺ dominates over the 2H⁺–Mg²⁺ substitution at pressures < 8 GPa, while the Al-coupled substitution dominates in clinopyroxene at all pressures. Our data permit a new evaluation of the maximum storage capacity of water in nominally anhydrous upper mantle peridotite and eclogite. The water storage capacity of peridotite increases gradually with pressure to a maximum of 0.6 wt.% H₂O at 410 km depth; the storage capacity of eclogite is 0.4 to 0.5 wt.% H₂O from 2–5 GPa, dropping gradually to ~0.2 wt.% just above the transition zone as majorite is formed at the expense of pyroxene. We show that the water abundances inferred for mid-ocean ridge and hotspot magma sources are not consistent with the composition of water-rich mantle residues emerging from a hydrous melt layer at the top of the transition zone. Regional variations observed in the H₂O–LREE systematics of oceanic basalts can result from derivation of these magmas from depleted mantle sources having polybaric melting histories, with high-H₂O/Ce sources being residues of shallow (garnet-absent) melting, and low-H₂O/Ce sources being the residues of deep melting in the stability field of garnet lherzolite.