The phase changes that occur as water invades a superheated geothermal reservoir were studied experimentally. Cold liquid ether was injected into a hot, isothermal bed of sand, and the rate of vaporization and thermal evolution of the sand bed were measured. When the ether was injected from below, the ascending vaporization front remained nearly planar. In accord with theoretical models, an isothermal layer of saturated liquid developed behind the front, and the mass fraction vaporizing was proportional to the reservoir superheat. In a second experiment, liquid was injected from below into a thermally stratified reservoir in which an initially superheated layer was overlain by a supercooled layer. As vapor produced at the ascending front rose into the supercooled region, it condensed and produced a two-phase saturated layer above the superheated zone, even though injection was from below. In a third experiment, liquid was injected from above into an isothermal superheated bed of sand. As the vaporizing liquid front migrated downward, hot vapor rose through the descending liquid. This vapor maintained the upper part of the reservoir close to the saturation temperature even though cold liquid was continuously supplied from above. In addition, fingers of liquid descended ahead of the main front into the superheated zone. Liquid transported in these fingers accumulated at the base of the reservoir in a layer of saturated liquid. Much of the subsequent vaporization occurred at the ascending front as the thickness of this liquid layer increased. Only the intermediate part of the reservoir remained superheated. Our experiments demonstrate that the geometry of recharge and the initial thermal structure of the reservoir have an important control on the phase changes in active geothermal systems. In many cases, recharge of liquid in a superheated region of a reservoir can lead to vapor production and transport and this may cause initially supercooled regions to become saturated. Also, if the liquid front becomes unstable, a two-phase zone may develop from an initially superheated region. ¿ American Geophysical Union 1996