Magmatic rocks from the Antarctic Peninsula show marked variations in isotope composition, which reflect changes in the geodynamic evolution of the peninsula through time. Most Antarctic Peninsula granitoids formed as a result of subduction: they fall on well‐defined trends on plots of ϵNd, 207Pb/204Pb and δ18O against 87Sr/86Sri, between a component derived from subduction‐modified mantle or juvenile basaltic underplate (ϵNdi>6, 207Pb/204Pb=15.61, δ18O=5.5‰, 87Sr/86Sr<0.704) and an end‐member interpreted as a melt of Proterozoic lower crust ( ϵNd=−7, 207Pb/204Pb=15.67, δ18O=10‰,87Sr/86Sr=0.709). A small group of granitoids, emplaced before or during Gondwana break‐up, plot on distinct trends towards high 87Sr/86Sri compositions, reflecting mixing between melts derived from Proterozoic lower crust and melts of middle–upper crustal rocks (ϵNdi=−9, 207Pb/204Pb=15.64, δ18O=10‰, 87Sr/86Sr=0.726), with little or no input of new material derived from the mantle or from juvenile basaltic underplate. These granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of Gondwana break‐up. Similar trends are shown by data from granitoids of the adjacent crustal blocks of West Antarctica. The isotope data suggest that an enriched Ferrar/Karoo‐type lithosphere was not involved in the genesis of granitoids of the Antarctic Peninsula or of the Ellsworth–Whitmore Mountains crustal block.