New Hf isotopic compositions for island arc basalts from the Luzon arc (Philippines) define a remarkable sub-horizontal trend in Hf–Nd isotopic space with a small range of Hf (+5 to +17) associated with a large variation in Nd (–7 to +8). The data plot above and barely overlap the terrestrial array defined by oceanic basalts and continental crust. Mixing hyperbolas passing through the data intersect fields for depleted mantle and pelagic sediments suggesting that these two components formed the source of the Luzon arc lavas. An exception is the Batan Island where the low Nd ratios are associated with low Hf values. A mixing hyperbola fitting the Batan samples suggests that their mantle source was modified by subducted material prior to contamination by terrigenous clays. More generally, the geochemical relationships in Luzon lavas show that the mixing endmembers are source components rather than melts. The relationship between Nd and Hf isotopic compositions in the Luzon volcanics show that the type of sediment subducted under an island arc is a determining factor in the control of the two isotopic systems in island arc environments.