THE 14 January 1993 eruption of Galeras volcano, in Colombia, which killed six scientists and three tourists(1), was followed by a larger eruption on 23 March 1993(2). Both eruptions were preceded by episodes of long-period seismicity. The source of long-period seismic events has been modelled extensively(3-8), as the resonance within a fluid-filled crack induced by pressure transients in the fluid(5-7). Here,ve use the SO2 flux from Galeras volcano, measured remotely, to infer the degassing history during the episode of long-period events preceding the 23 March eruption. SO2 flux and long-period seismicity have been monitored separately elsewhere to forecast volcanic activity(9-13). Our results show how the combination and correlation of the two methods can be used to interpret the movement of gases from the magma body to the surface, and to monitor the pressure buildup leading to an eruption at explosive volcanoes.