As part of a comprehensive study of Floreana volcano in the Galapagos, multiple samples were collected from several single eruptive units to test for intra-eruptive compositional variation. Floreana is unique in the Galapagos for having especially explosive basaltic volcanism, primitive alkali-olivine basalts, and the compositional imprint of mantle metasomatism. The eruptive units that were sampled span the age of the exposed surface lavas of Floreana. Some of the units display substantial trace and major element variation, whereas others are nearly homogeneous. Specifically, indications are that some of the youngest eruptive events on Floreana, including the Alieri cone and vent system and the Cerro Pajas cone, originate from a relatively homogeneous mantle source (e.g., Zr/Nb: 3.3-3.7) and the lavas show little effect of shallow fractionation (MgO: 12.1-12.8 wt.%). In contrast, the slightly older Pajas flow, an expansive lava field that covers much of the western flank of the volcano, exhibits a wider range in both MgO (7.2-9.4 wt.%) and incompatible trace element contents. The Pajas flow is the most evolved surficial suite observed on an island otherwise constructed of primitive basalats and one of the few with evidence of clinopyroxene fractionation. Our mapping correlates this flow to Cerro Pajas, and it shows a systematic variation in parameters including La/Ce and Sc/Y. An increase in the extent of partial melting is indicated if the Pajas flow and cone units are related, though isotopic data and further fieldwork are needed to test this hypothesis. A reversely-polarized vent from the eastern sector of the island is made up of lavas whose incompatible trace element ratio variation (e.g., Zr/Nb: 2.2-2.5) is less than that of the Pajas flow (Zr/Nb: 4.2-5.3), but the range in major element contents is comparable (MgO: 12.7-15.9 wt.%). These variations are likely the result of disaggregation of wehrlite xenoliths. Samples taken from a reversely-polarized lava flow from western Floreana, whose correlation is somewhat uncertain, range in MgO from 11.2-14.8 wt.% and display a more pronounced and variable effect of what we attribute to be mantle metasomatism. Thus, at Floreana the development of compositional variation over the timescale of a single eruption can be significant and is the result of a combination of effects including shallow fractionation, extent of partial melt generation, incorporation of mantle xenoliths, and metasomatism, whose contributions vary from one eruptive unit to another.