In many phase transformation processes, natural convection controls the freezing or melting rate of the material. The kinematics of the solid-liquid interface, which are coupled with bulk convection in the melt, play an important role in determining the microstructure of a solidified :material. Although the fundamental problem of thermoconvective instability of a single-component in a horizontal liquid layer has been studied extensively, there still exist additional complexities which arise during solidification due to the presence of nonmelting components. This study addresses the problem of Rayleigh-Benard instability of a liquid layer in the presence of suspended particles undergoing a phase transformation. A linear stability analysis determined the effects of the particles and phase-change on the conditions for incipient convection. The analysis reveals that the concentration of and heat transfer between particles affect the stability of the system. (C) 2000 Elsevier Science Ltd. All rights reserved.