Since late 1994, open-vent eruptive activity and degassing at Popocatépetl volcano, Mexico, have released large masses of CO2 and SO2. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. We present the first measurements of dissolved CO2 in the mafic magma end member based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as ~400 MPa (~15 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ~150–350 MPa pressure can explain the CO2/SO2 mass ratios (1–8) of volcanic gases released from the volcano during 1995–1997. Our results demonstrate that mafic magma recharge was responsible for the high measured fluxes of CO2 and SO2 from 1995 to 1997. The total SO2 emission of 9 Mt during this period requires intrusion and degassing of a minimum of 0.8 km3 of mafic magma. Only ~0.3% of this new mafic magma has been erupted in the form of mixed (hybrid) lava and tephra. Our results suggest that the ongoing eruption of Popocatépetl is essentially an intrusive event. More generally, we suggest that intrusion and deep degassing may explain the high gas fluxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits.