The first measurement of 18O/16O ratios of plagioclase, clinopyroxene, orthopyroxene, and titanomagnetite phenocrysts from modern Aleutian island-arc lavas provides new insight and independent constraints on magma sources and intracrustal processes. Basalts are heterogeneous on the scale of the entire arc (δ18Oplag = 5.5‰-6.7‰) and individual volcanic centers (ranges of 0.3‰ to 0.8‰). Combined with Sr isotope and trace element data, δ18Oplag values suggest a variable magma source characterized by differences in the mantle wedge or the subducted sediment component along the volcanic front. Seven tholeiitic basalt to rhyodacite (50%-71% SiO2) lavas from the Seguam volcanic center have nearly identical δ18Oplag values of 6.0‰ ±0.2‰, reflecting extensive closed-system plagioclase-dominated crystal fractionation. Oxygen isotope thermometry and pyroxene and oxide equilibria indicate that differentiation occurred between 1150 ±100 °C (basalt) and 950 ±100 °C (rhyodacite). In contrast, δ18Oplag values of 12 calc-alkalic basaltic andesites and andesites (53%-62% SiO2) from the smaller Kanaga volcanic center span a broader range of 5.9‰-6.6‰, and consist of mostly higher values. Isotopic disequilibrium in the Kanaga system is manifest in two ways: two types of basaltic inclusions with contrasting δ18O values (6.0‰ and 6.5‰) occur in one andesite (δ18Oplag = 6.6‰), and in two other andesites plagioclase-titanomagnetite and clinopyroxene-titanomagnetite oxygen isotope temperatures are inconsistent. One andesite, however, yields concordant oxygen isotope temperatures of ∼850 ±100 °C, and a two-pyroxene temperature of 920 ±50 °C. Assimilation of isotopically heavy crustal wall rock and mixing off basaltic and andesitic magmas explains the variable and high δ18Oplag values and low eruptive temperature of the Kanaga andesites.