Explosive volcanic eruptions are triggered when magma is sufficiently overpressured to open up a fracture to the surface and the eruption continues until this overpressure is relieved. Here we show that the mass erupted from the chamber during the eruption depends critically on whether the magma becomes saturated in its volatiles. Unsaturated liquid magma may only deform elastically and is therefore relatively incompressible. Thus the eruption of relatively small fractions, 0.001--0.01, of the magma in the chamber is sufficient to relieve the chamber overpressure. In contrast, if the magma is saturated and contains exsolved bubbles of vapor, then the mixture becomes much more compressible and the chamber overpressure is only relieved when a fraction of order 0.01--0.1 of the initial mass in the chamber has erupted. In such cases the mass of exsolved bubbles of vapor in the magma as well as the depth and vertical extent of the chamber have a dominant control on the mass erupted. It is also shown that for a given total volatile content, since the mass of volatiles exsolved as gas increases with the crystal content of the magma, the mass erupted also tends to increase with magmatic crystal content. In a layered chamber the volatile content of each layer and the depth and vertical extent of the chamber again exert the main controls on the mass erupted. We use our model to interpret chamber sizes in a number of well-documented historic eruptions.¿ 1997 American Geophysical Union