Okmok volcano is situated on oceanic crust in the central Aleutian arc and experienced large (∼ 15 km3) caldera-forming eruptions at ∼12 000 years bp and 2050 years bp. Each caldera-forming eruption began with a small Plinian rhyodacite event followed by the emplacement of a dominantly andesitic ash-flow unit, whereas effusive inter- and post-caldera lavas have been more basaltic. Phenocryst assemblages are composed of olivine + pyroxene + plagioclase ± Fe-Ti oxides and indicate crystallization at 1000-1100°C at 0.1.0.2 GPa in the presence of 0-4 H2O. The erupted products follow a tholeiitic evolutionary trend and calculated liquid compositions range from 52 to 68 wt % SiO2 with 0.8-3.3 wt % K2O. Major and trace element models suggest that the more evolved magmas were produced by 50-60 in situ fractional crystallization around the margins of the shallow magma chamber. Oxygen and strontium isotope data (δ18O 4.4-4.9%, 87Sr/86Sr 0.7032-0.7034) indicate interaction with a hydrothermally altered crustal component, which led to elevated thorium isotope ratios in some caldera-forming magmas. This compromises the use of uranium-thorium disequilibria [(230Th/238U) = 0.849-0.964] to constrain the time scales of magma differentiation but instead suggests that the age of the hydrothermal system is ∼100 ka. Modelling of the diffusion of strontium in plagioclase indicates that many evolved crystal rims formed less than 200 years prior to eruption. This addition of rim material probably reflects the remobilization of crystals from the chamber margins following replenishment. Basaltic recharge led to the expansion of the magma chamber, which was responsible for the most recent caldera-forming event.