Experimental studies of two-phase thermal convection in a porous medium have been carried out in a sandbox. An initially water-satured porous layer was heated from below and cooled from above. When the lower boundary of the layer reached 100¿ C, a two-phase steam-water zone developed. The variable parameters in this study were the thickness of the porous layer and the heat flux. Measurements indicated that there can exist a stable form of two-phase convection in a porous medium. Polyhedral convection cells were observed in the water-saturated zone above the two-phase zone. The dimensions of the cells varied with the heat flux for a given thickness. The two-phase region was of variable geometry and had a distinctly nonhorizontal steam-water interface with the overlying water zone. Convective instabilities were initiated by the occurrence of the phase change. At high heat fluxes a dry steam zone developed in which temperatures exceeded 110¿ C. The dry steam zone was located close to the base plate with a wet steam zone above. A one-dimensional heat and mass transfer model is used to analyze the experimental data. The results are compared to observations made on vapor-dominated geothermal systems and applied to some mineralization problems.