SUMMARYAn accurate computational analysis is presented for the onset of thermal convection in a two-layer system which is comprised of a saturated layer of porous material described by Darcy's law, over which lies a layer of the same saturating #uid. The two-layer system is heated from below and the upper (#uid) surface is allowed to be "xed or stress free. The onset of convection may have a bi-modal nature in which convection may be dominated by the porous medium or by the #uid depending on the depths of the relative layers, but this is strongly controlled by material parameters. The e!ect of variation of relevant #uid and porous material properties is investigated in detail, as is the e!ect of the interface boundary condition between the #uid and the porous medium.