The eastern Sunda and Banda arcs are characterized by an anomalous tectonic setting when compared with most other island arcs. Although the volcanic arc appears to be underlain by oceanic crust, the trough and sea floor beyond appear to be underlain by relatively thick continental crust. Late Cenozoic basaltic volcanic rocks from Solor in the west to Serua in the east are characterized by a primitive tholeiitic geochemistry, whereas associated more silicic rocks are anomalously rich in alkalis, especially potassium. 87Sr/86Sr ratios are all high relative to those from the western Sunda arc; they range from 0.7054 (Pura Beser) to 0.7091 (Maupura and Serua). Subduction and subsequent involvement in magma genesis of either sediments or continental crust seems the most plausible explanation. This model may also explain some of the tectonic peculiarities of the region, including the present gap in active volcanism north of Timor and the strong Holocene vertical uplift in the region.