We investigate the solutions of partial differential equations describing a two-layered fluid system. The lower layer contains a fastly diffusing, stabilizing substance T and a slowly diffusing, destabilizing substance S. The upper layer contains only the solvent. Double-diffusive convection yields cells in agreement with experiments performed with a tenside (S) and glycerine (T) dissolved in water. Approximation of the PDEs renders scaling laws for the appearance time T em of the cells and their wavelength λ, as functions of the initial concentrations S 0 and T 0 , as well as λ as a function of time. These approximations agree well with the solutions of the PDEs.