U-series isotopes can provide unique insights into the physical processes of magma evolution by constraining the time scales over which they operate. This, however, requires rock suites that provide a clear and complete record of the liquid line of descent. Sangeang Api volcano, in the east Sunda arc, provides such an opportunity because it erupts potassic lavas (SiO2∼47-55%), which contain a spectrum of xenoliths interpreted to represent the cumulates complementary to the lavas. Combined, the cumulates and lavas span a large compositional range, which major and trace element modelling suggests reflects ∼70% polybaric crystallization that began at sub-Moho depths and continued into the upper crust. The parental magmas can be successfully modelled by ∼3% dynamic partial melting, in the presence of 0-4% residual garnet, of a mid-ocean ridge basalt (MORB) source enriched by ∼3% subducted Sunda sediment in addition to a contribution of fluid-mobile elements from the subducting slab. The effects of fluid addition and partial melting on U-Th disequilibria appear to be competing processes. A moderate range in Sr, Nd and Pb isotopes is interpreted to reflect a combination of source heterogeneity and <15% assimilation of Indian MORB crust. Neither interaction with metasomalized arc lithosphere nor the presence of enriched, plume-type mantle in the mantle wedge is required by our data. The cumulates and lavas have largely indistinguishable (230Th/232Th) over a wide range of U/Th ratios and thus any age differences are minimal relative to the half-life of 230Th. All whole rocks and minerals are characterized by 226Ra excesses and modelling of the 226Ra-230Th-Ba data suggests that magmatic evolution beneath this arc volcano occurs on time scales of ∼2000 years. Combining this with simple numerical calculations we estimate that the Sangeang Api magma chamber is ∼6-10 km3 in size, cooling rates are ∼0.05°C/yr and minimum crystal growth rates are (2-7) × 10-13 cm/s. It is possible that a significant proportion of the crystal growth and differentiation occurred during isobaric decompression of magmas ascending through conduits. An implication is that the net magmatic flux across the Moho is of magmas that are already significantly evolved from primary magmas and this may be significantly for why average continental crust has an andesitic bulk composition even though the flux out of the mantle wedge is basaltic.