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INTRODUCTION Fissures sliced through Kīlauea Volcano’s lower east rift zone on 3 May 2018, eventually engulfing hundreds of structures in lava flows and triggering a collapse at the summit. During the eruption, we employed a rapid routine for geochemical analysis of lava, developed over 6 years of monitoring the prior continuous eruption at Kīlauea. The application of this routine elevated lava chemistry to a near real-time data stream in eruption monitoring, similar to seismic and geodetic data. It provided an unparalleled opportunity to understand changes in magma characteristics during a rapidly evolving eruptive crisis. RATIONALE Lava chemistry provides vital information on the underground sources of magma, eruptive conditions, temperature, and physical properties of lava flows. However, analytical techniques are typically slow, leaving chemical analysis of lava as a retrospective tool in the volcano sciences. We developed an analytical procedure to characterize the geochemistry of lava within a few hours of sample collection, allowing us to identify a specific suite of major and trace elements that track lava compositions and estimate lava temperatures through chemical geothermometers. This information was used to inform response teams of shifts in eruptive conditions. RESULTS The initial fissures erupted low volumes of chemically evolved basaltic lavas from 3 to 9 May, which were viscous and cool (~1110°C). On 13 May we detected less-evolved compositions and an increase in inferred lava temperatures (~1130°C). We informed science and response teams that the arrival of more fluid and voluminous lava was likely. Beginning 17 to 18 May, the lava from the primary fissures became increasingly less chemically evolved, hotter, and more fluid. By 28 May, activity focused on a single vent (fissure 8). This vent fed a massive outpouring of hotter (~1145°C) lava that continued for more than 2 months. During this stage, lavas became slightly hotter and lost the cargo of lower-temperature minerals that were initially abundant. The lava carried olivine crystals with unusually high MgO, indicative of the presence of much hotter magma (>1270°C) somewhere in the plumbing system. A second dominant olivine population formed in cooler magma similar to what was being erupted previously at the summit lava lake. We also identified simultaneous, but more explosive, repetitive outbursts of andesite lava. This highly viscous and evolved composition, not previously known from Kīlauea, erupted at low temperatures (1060° to 1090°C) on a fissure offset from the other eruption fissures. The chemical and mineralogical fingerprint of this lava was also detected at other fissures several kilometers from the andesite vent. CONCLUSION Analysis of the data during the eruption revealed that at least three different sources of magma were feeding the eruption. The first two were the chemically evolved basalt of the initial fissures and the highly viscous andesite. Both are volumetrically minor sources that represent distinct pockets of old residual magma from Kīlauea’s east rift zone that evolved for more than 55 years, cooling and crystallizing at depth. The third and volumetrically more substantial source was less-evolved and hotter basalt of fissure 8. This source was similar in composition to the magma erupted at Kīlauea in the years before 2018 and was ultimately derived from the summit region. Draining and collapse of the summit by this voluminous eruption may have stirred up deeper, hotter parts of the summit magma system and sent mixed magma down the rift. By the final 20 days of the eruption, most magma stored within the active rift system had flushed out. Posteruption analyses done by traditional geochemical methods confirmed that the rapid-response routine produced comparable data and validated the models proposed during the active eruption. Our work has demonstrated that geochemical analyses of lava samples in near-real-time can yield critical information that enhances hazard assessments and risk mitigation during an eruption.