A linear stability analysis has been implemented for a viscous flow induced by internal heat sources in a vertical annular porous region bounded by two concentric cylinders. The analysis is made under the assumption that the flow is parallel to the cylinders. The perturbation equations are solved by a Chebyshev collocation spectral method. Results are given for a wide range of gaps. The critical curves in several planes formed by the physical parameters of the problem are plotted. In particular, the effects of the porous parameter and the radius ratio are examined. (C) 2003 Elsevier Ltd. All rights reserved.