Experiments were conducted to explore the behavior of convection and heat transfer during phase separation in porous media. Phase separation is considered to be an important process in many mid-ocean ridge hydrothermal systems in which a dense brine separates from a lighter, less saline vapor phase at supercritical temperatures. As this process occurs at high temperatures and pressures in seawater systems, a binary fluid (H2O-2-butoxyethanol) with a lower consolute point of 48.5¿C, was used as a proxy for the hydrothermal fluid. In the experiments, a cell containing glass beads saturated with the binary fluid was heated from below and cooled from above. It was demonstrated that chemically differentiated regions can exist as part of a steady state convective regime, with the denser fluid phase separating and accumulating in a stagnant bottom layer. Furthermore, in upwelling regions phase separating fluids can become entrained. Phase separation was also found to lower the efficiency of convective thermal transfer, indicated by the deviation from the calculated single-phase Rayleigh-Nusselt curve. It is proposed that the reduction of convective heat transfer at supercritical conditions may be a crucial factor in controlling heat transfer through the oceanic crust.