Northwestern Palmer Land is situated within the Mesozoic magmatic arc of the Antarctic Peninsula. Three temporally distinct granitoid groups, of lower Paleozoic, Jurassic, and Cretaceous ages, have been identified in this region. They are all calc-alkaline, I-type granitoids, ranging from quartz-diorites to granites, have high LIL/HFS element ratios, and their analyses all fall in the volcanic arc granite field when plotted on granite discrimination diagrams. The Paleozoic group comprises orthogneisses that have undergone high-grade metamorphism. The Jurassic group is variably foliated and partially recrystallized. It is subdivided into high-K metagranitoids and a smaller subgroup of low-K metatonalites, which are restricted to the western margin. The Cretaceous granitoids, which are undeformed, are subdivided on the basis of petrography into granophyric granites and nongranophyric granitoids. The orthogneisses, high-K metagranitoids, and nongranophyric granitoids are characterized by increasing LIL elements (particularly K and Rb) with increasing SiO₂ and an initial increase followed by a decrease in the HFS elements (Zr, Nb, and, to a lesser extent, Y). Variable degrees of partial melting or fractional crystallization could have produced the mafic to intermediate compositions, but plagioclase, K-feldspar, and minor phase fractionation are most likely responsible for the geochemical trends in the intermediate to acid compositions. In contrast the metatonalites show no enrichment in either the LIL or the HFS elements but still have high LIL/HFS element ratios. Their origin is uncertain, although they appear to have been derived from the same source as the high-K metagranitoids, but probably evolved under higher P_H2O and f_O2 conditions. The Cretaceous granophyric granites are more enriched in both the LIL elements and the HFS elements at high SiO₂ levels than the nongranophyric granitoids. These are interpreted to have differentiated from the nongranophyric granitoids by the process of filter-pressing. Rb-Sr isotope data for all the groups suggest there is no correlation between initial ⁸⁷Sr/⁸⁶Sr ratios and time. The values lie in the range 0.705 to 0.707, suggesting that all the groups are predominantly mantle-derived but may contain a significant crustal component. The orthogneisses are part of the extensive Paleozoic plutonism that occurred along the length of the Pacific margin of Gondwanaland. Within the context of the Antarctic Peninsula as a whole, the Jurassic granitoids are geographically more restricted and geochemically more variable than the Cretaceous granitoids. The latter form a geochemically uniform group of plutons that occur throughout the Antarctic Peninsula.