We develop a problem recently studied by Postelnicu in which a horizontal layer of porous material is subject to a chemical reaction at the base of the layer. The porous medium is of Darcy type and the layer is saturated with a non-isothermal liquid containing a concentration of the reacting chemical. The equations are developed and then it is shown that if the reaction parameter B of Postelnicu exceeds a critical value convection commences but as oscillatory convection, not stationary convection, as previously thought. (C) 2011 Elsevier Ltd. All rights reserved.