The Darcy model with Bousinesq approximation is used to study double-diffusive natural convection in a horizontal porous layer subjected to vertical gradients of heat and solute. Results are presented for 0 ≤ R_T ≤ 100, -1 ≤ N ≤ 50,10^-2 ≤ Le ≤ 10² and 5 ≤ A ≤ 10, where R_T, N, Le and A correspond to the thermal Rayleigh number, buoyancy ratio, Lewis number and aspect ratio of the enclosure, respectively. An approximate solution is obtained by assuming parallel flow in the core region of the cavity and a numerical solution by solving the complete governing equations. The critical Rayleigh number for the onset of thermoso-lutal convection in infinite porous layer is predicted. The results for heat driven flows (N → 0) and solute driven flow (N → ∞) emerge from the present analysis as limiting cases. The agreement between the analytical and numerical solution is shown to be satisfactory.