Small-angle x-ray scattering analysis of concentration fluctuations in polystyrene-polybutadiene mixtures is interpreted using a recently derived mean-field version of the gradient term of the Landau expansion for the free energy. Available information on the temperature dependence of the interaction parameter, obtained from optical determination of cloud point curves, allows direct comparison between theoretical and experimental correlation lengths at temperatures approaching the spinodal line in the homogeneous phase. Critical wave vectors after quenching below the spinodal line are analyzed in similar fashion. Polydispersity effects on the spinodal line and the structure factor are calculated for an arbitrary shape of the molecular weight distribution. The values of the temperature coefficient of obtained separately from cloud point curves and scattering data near the spinodal line are consistent. Comparison after quenching to temperatures below the spinodal line shows systematic overestimation of critical wavelengths. © 1987 The American Physical Society.