Research on thermohaline convection in porous media has borrowed heavily from the field of viscous liquid thermohaline convection. Transient thermohaline convection in porous media differs dramatically from that in viscous liquids because of thermal retardation. The effects of thermal retardation are well known in geothermal reservoir engineering for forced convection processes such as liquid reinjection. Thermal retardation causes heat to move at approximately the Darcy velocity, while solute moves at the pore velocity. The difference in transport of heat and solute makes transient thermohaline convection in porous media double-advective rather than double-diffusive. For upward moving plumes, dense solute is advected in front of the thermal plume and creates a density lid. For downward moving plumes, the dense solute plume separates from the thermal plume. ¿ 1999 American Geophysical Union