Abstract SO2 flux and gas composition were measured during the 2021 flank eruption of Pacaya Volcano (Guatemala). The SO2 flux, measured with Tropomi, gradually rose from βΌ2 to βΌ30 kg/s over a 5 month period before the eruption. Field UV camera data showed that >95% of the SO2 was emitted from the summit crater. The Multigas composition measurements evidenced a depletion in CO2 and a significant enrichment in H2O over SO2 in the plume from the flank eruptive fissure compared to the central crater plume, and punctual amounts of H2S. We explain these variations by a combination of (a) Fractional degassing between a vertical conduit and an almost horizontal dyke diverging at a calculated pressure of 10β20 MPa, (b) SO2 scrubbing by a local and temporary hydrothermal system, and (c) Inclusion of groundwater in the magma dyke, even if visually the activity looked purely magmatic (lava spattering and effusion).