The nine diffusion coefficients that describe the isothermal transport in the system poly(ethylene glycol) = 200 uma)-poly(ethylene glycol)(M-w = 200 uma)-NaCl-water have been measured at one mean composition with the Gouy interferometric technique. The gravitational instability of the diffusion boundary has been verified, and the appearance of double diffusive convection in agreement with the fluid-dynamic theory was observed. In particular, it was possible to observe the simultaneous appearance of fingers and the so-called over-stability at the diffusion boundary. This result points out the fact that diffusion in multi-component systems may be responsible for the evolution of dissipative,structures that increase their complexity as the number of components in the solution increases. Furthermore, it calls the attention on the possibility to simulate, in controlled laboratory conditions. the evolution of stratifications found in the ocean areas, responsible for the transport of energy and mass through the sea water. (C) 2004 Elsevier Ltd. All rights reserved.