The article deals with the phase diagram and spinodal decomposition of two polymers blends, namely d—PS/PVME and d(deutero)—PS/PS, investigated by small angle neutron scattering (SANS). The result of the static experiments is the excess free energy and the phase diagram. This is used as a basis for studies of non—equilibrium phenomena as spinodal decomposition. In polymer blends the Cahn—Hilliard—Cook theory of the early state of spinodal decomposition can be tested easily, because the blends have rather low relaxation rates; and they are meanfield systems [2,6] which makes interpretation simple, except in a very narrow temperature region near the critical point . The kinetics in the isotopic blend d—PS/PS are so slow that the early states of spinodal decomposition can be studied within minutes. The presented SANS results have been performed at the KWS I small angle instrument at the FRJ—2 reactor in the KFA Jülich.