A straightforward non-linear extension of Boussinesq’s approximation for two-component fluids is presented. The perturbative method proposed permits the numerical separation of the different orders in the obtained hierarchy of equations. The procedure is useful for the analysis of buoyancy driven flows and the stability of convective patterns. The fluid layer thicknesses, for which the Sorer and Dufour effects must be retained in the equations, may be determined. The influence of the hydrostatic field on the heat equation may be obtained in relation to the layer thickness. The analysis permits us to obtain useful conclusions about the stability in two important examples of this kind of system : dry air and salt water. The method may be easily extended to multi-component fluids and to any other physical problem.