The andesite lava erupted at the Soufrière Hills Volcano (SHV) is crystal-rich with 33–63% phenocrysts of plagioclase (65%), amphibole (28%), orthopyroxene (7%), and minor Fe–Ti oxide and clinopyroxene microphenocrysts. The andesite hosts mafic enclaves that have similar mineral phases to the andesite. The enclaves are generally crystal-poor but can have up to 27% of inherited phenocrysts from the andesite, the majority of which are plagioclase. The eruption is defined by discrete periods of extrusion called phases, separated by pauses. The enclaves exhibit bulk geochemical trends that are consistent with fractionation. We infer that the intruded mafic liquids of Phases I and II interacted and assimilated plutonic residue remaining from the multiple prior mafic intrusions, while the basaltic liquids from Phases III and V assimilated relatively little material. We also infer a change in the basaltic composition coming from depth. The bulk Fe contents of both magma types are coupled and they both show a systematic interphase variation in Fe content. We interpret the coupled Fe variation to be due to contamination of the andesite from the intruding basalt via diffusion and advection processes, resulting in the erupted andesite products bearing the geochemical imprint of the syn-eruptive enclaves.