Sm-Nd and Rb-Sr data are presented for six volcanic and volcaniclastic samples from the Eocene belt and the Toa Baja drill hole of Puerto Rico in order to provide a preliminary isotopic description of Greater Antilles arc rocks. The lava samples have trace and rare earth element compositions typical of island arc rocks. The isotopic compositions of the volcaniclastic samples are displaced from those of the lavas, and suggest that they include an authigenic or detrital crust component. The similarity of Sr and Nd isotopic ratios of Puerto Rican lavas and northern Lesser Antilles volcanic rocks suggests that they may have been produced from similar magma sources. ¿American Geophysical Union 1991