The Peninsular Ranges of southern and Baja California are divided into a western, predominantly magnetite-bearing plutonic subprovince and an eastern, predominantly magnetite-free plutonic subprovince. The boundary that separates the two subprovinces corresponds roughly to the southwestern margin of the La Posta superunit, but in some places extends into the La Posta granitic province. Neither the pre–La Posta foliated granitic rocks nor the garnet- or muscovite-bearing rocks of the eastern Peninsular Ranges contain magnetite. The magnetite/ilmenite distinction occurs on three scales: regional variations that appear to be independent of host rock or individual plutons, variations paralleling modal facies within zoned plutons, and contact loss of magnetite in the outer margin of a pluton (from meters to more than a kilometer in width). Observations to date indicate that the regional distribution of magnetite- and ilmenite-series granitic rocks may result from generation of parental magma within the dehydration zone of a subduction plane. The gradation within zoned plutons probably results from a lowering of oxygen fugacity in the magma during progressive crystallization. The contact effect appears to be a consequence of reactions between the cooling pluton, the host rocks, and water-rich fluids from a variety of sources.