It is chemically inappropriate to call many of the rocks in island arcs calc-alkaline and we suggest they be known as the “island arc tholeiitic series”. They differ from calc-alkaline rocks by having a lower silica mode, more iron-enrichment, higher Na 2O/K 2O ratios, less K and associated trace elements, K/Rb∼ 1000,Th/U∼ 1–2, and chondritic REE patterns with La/Yb= 1–2. When erupting concurrently, rocks with these characteristics occur shallower earthquake foci on the trenchward side of island arcs than do the calc-alkaline. They and not rocks of the traditional calc-alkaline series are the most dominant in many western Pacific and Atlantic Island arcs and represent the earliest stages in arc evolution. Although sharing some tholeiitic features, they differ from normal tholeiitic series by having a higher percentage of intermediate and acid members and too little normative olivine, for example, to have been in equilibrium with peridotite. In such distinctive features as REE, Th, and U contents and La/Yb, Th/U, and K/Rb ratios, they are more like mid-ocean ridge tholeiites than any other terrestrial rocks, but they differ from them in silica mode, alkali content and isotopic composition, and MgO, FeO, Ni, and Cr contents. Some of their spatial, temporal, and chemical characteristics may be explained by mixing of the partial melting products of resorbed lithosphere and overlying upper mantle. We report REE data for 19 samples, chosen to highlight the different patterns found in modern and ancient island arcs.