The partial melting, along appropriate geotherms, of a descending slab of oceanic crust is examined in the light of recent experimental studies. The predicted compositions of magmas generated by this model are concurrent with those observed in island arcs. It is suggested that magmas of the island arc tholeiitic series are produced by reactions involving the breakdown of amphibole at shallow depths. At greater depths partial melting of eclogite will generate calc-alkaline magmas. Between these two extreme types a continuum of transitional magmas could be generated.