Laminar, steady and parallel buoyant flow in a concentric vertical annulus filled with a porous medium is studied. The Darcy-Brinkman model and the Oberbeck-Boussinesq approximation are employed. The governing equations are written in a dimensionless form and solved analytically by means of the variation of constants method. In order to investigate the onset of flow reversal, both at the inner and at the outer boundary, a dimensionless reference temperature 0(o) is introduced. Moreover, this quantity is related to the usual GrIRe parameter. The limiting cases of Darcy flow and clear fluid are investigated as well. (C) 2010 Elsevier Ltd. All rights reserved.