The order parameter in binary homopolymer blends or diblock copolymers describes the deviations of the local composition of both components from the mean value. The Fourier transform of the fluctuation spectrum yields the structure factor S(Q), where Q is the wave number of the fluctuations or the momentum transfer in scattering. S(Q→O) is the susceptibility related to the second derivative of the Gibbs potential of mixing with respect to the composition, t32AG/t32z [1–3]. Binary polymer blends normally reveal a miscibility gap that separates the homogeneous one-phase from the two-phase region where precipitation appears. Approaching the critical temperature Tc, the strength and range of thermal fluctuations of the order parameter increase and diverge if Tcis approached.