An important type of gravitational lava-dome failure involves significant internal gas overpressure that influences the failure process. For the first time, this paper presents a rigorous analysis of the mechanics of gas-pressurized dome failure. We develop diffusion models to calculate gas overpressures in a lava dome, and embed these pressure data into stability analyses to demonstrate how gas pressurization can initiate instability. The results bear on the mechanistic understanding of hazardous dome failures. Gas pressurization can promote deep-seated failure, and the timing of conduit pressurization can control the timing of dome collapse. Theory explains why dome collapse can be delayed for cases of oscillating effusion. Dome collapses and explosive events linked to collapse of lava domes can be hazardous. Monitoring criteria are considered to anticipate whether or not such collapses are likely to be explosive. ¿ 2000 American Geophysical Union