This study deals with the onset of thermal natural convection in a system consisting of a fluid layer overlying a homogeneous porous medium. A linear stability analysis is carried out, using the so-called two-domain approach and including the Brinkman term in the porous region (2 Omega(DB)). Results are systematically compared to those obtained using the one-domain approach (1 Omega) and the classical Darcy formulation of the two-domain approach (2 Omega(D)). A better agreement is found between the 2 Omega(DB) and 2 Omega(D) neutral curves, than with the 1 Omega curves, indicating that the inclusion of the Brinkman term plays a secondary role on the stability results. The different treatment of the interfacial region is discussed on the basis of these results. (c) 2006 Elsevier Ltd. All rights reserved.