This paper reports the results obtained by means of scale analysis of two dimensional double diffusive natural convection in a square porous cavity isothermally heated from below and cooled from the upper surface, whereas its vertical boundaries are subjected to a horizontal solutal gradient. In the problem formulation, the Darcy model is considered with the Boussinesq approximation. The analysis is performed on the basis of the scaling law approach to predict the order of magnitude heat and mass transfer characteristics in limit cases corresponding to thermal or solutal driven flows. The results of the scale analysis are validated numerically and useful correlations of Nu and Sh, are proposed for wide ranges of the governing parameters, which are the thermal Rayleigh number, Ra-H, the Lewis number, Le and the buoyancy ratio, N. (C) 2004 Elsevier Ltd. All rights reserved.