Small-angle neutron scattering (SANS) experiments have been performed for three polybutadiene/polystyrene (dPB/PS) blends of differing dPB microstructure as a function of pressure and temperature. The experimental effective SANS interaction parameters are analyzed using the mean-field lattice cluster theory (LCT). In order to provide a meaningful comparison with the LCT, contributions from the non-mean-field long-range composition fluctuations are removed from the experimental data by use of a crossover function that describes the transition between near-critical and mean-field behaviors for the extrapolated zero-angle scattering. The theory provides a good description of the overall pressure dependence of the effective interaction parameter and its small dependence on the percentage of 1,2 addition units in the dPB chains. (C) 2001 American Institute of Physics.