In many solid/liquid phase transformation processes, natural convection controls the freezing or melting rate of the material. Freezing or melting in the presence of nonmelting components is important in numerous industrial processes and is also present in a wide range of systems in nature. The kinematics of the solid-liquid interface, coupled with bulk convection in the melt, play an important role in determining the microstructure of a solidified material. The fundamental problem of thermoconvective instability of a single-component fluid in a horizontal porous layer has been studied extensively; additional complexities arise during solidification due to the presence of a non-melting component, namely the porous medium. This study addresses the problem of Rayleigh-Benard instability of a liquid layer undergoing a phase transformation within a porous medium. A linear stability analysis is performed to determine the effects of the medium and phase-change on the conditions for incipient instability under a range of thermal boundary conditions. The analysis reveals that the onset of convection, or the stability of the system, is significantly affected by the presence of the porous medium, state of solidification and the thermal boundary conditions.