The validity of the local thermal equilibrium assumption in natural convection over a vertical flat plate embedded in porous medium is investigated analytically. The study is based on the two-phase (Schumann) model, using the Brinkman term (no-slip condition) to cover the now. It is found that there are four dimensionless parameters controlling the local thermal equilibrium assumption: the volumetric Biot number (Bi-v), the modified Rayleigh number (Ra-m), the modified Darcy number (Da) and the ratio of effective to dynamic viscosity (lambda). The effects of these parameters are investigated and a correlation equation is developed in order to determine the region where the local thermal equilibrium assumption is valid. (C) 2003 Elsevier Ltd. All rights reserved.