Minor Pliocene dacites from Crommyonia mark the western end of the South Aegean volcanic arc. They form small lava domes and flows generally associated with extensional faults. An older group (3.6–4 Ma) occurs in the west and a younger group (2.3–2.8 Ma) in the east. Volcanic rocks of similar age are found at Aegina, Poros and Milos in the western part of the South Aegean arc, whereas volcanism in the eastern part of the arc is of Quaternary age. The two groups of rocks at Crommyonia are chemically distinct. Both groups contain multiple generations of plagioclase. Both have εNd (−8.0 to −10.6) that is much more negative than any other rocks in the South Aegean arc and model ages that are similar to those for many Miocene extensional granites of the Cyclades. The model ages are interpreted to reflect a mid-Proterozoic mantle event recognized elsewhere in the Hellenides. The Crommyonia dacitic magmas represent the first stages of melting of deep lithosphere as a result of both subduction-related hydrous fluids and extensional decompression. Plagioclase compositions suggest important magma evolution in a base-of-crust magma chamber, where the strong crustal Nd isotope signature was acquired. With time, asthenospheric sources that upwelled as a result of extension played an increasingly important role in determining the isotopic characteristics of the arc volcanism.