Forecasting volcanic eruptions is critical to the mitigation of hazards for the millions of people living dangerously close to active volcanoes. Volcanic gases collected over five years from Galeras Volcano, Colombia, and analyzed for chemical and isotopic composition show the effects of long-term degassing of the magma body and a gradual decline in sulfur content of the gases. In contrast, short-term (weeks), sharp variations are the precursors to explosive eruptions. Selective absorption of magmatic SO2 and HCl due to interaction with low-temperature geothermal waters allows the gas emissions to become dominated by CO2. Absorption appears to precede an eruption because magmatic volatiles are slowed or retained by a sealing carapace, reducing the total flux of volatiles and allowing the hydrothermal volatiles to dominate gas emissions. Temporal changes in gas compositions were correlated with eruptive activity and provide new evidence bearing on the mechanism of this type of “pneumatic” explosive eruptions.