The Flory−Huggins interaction parameter, χ, for a series of critical binary blends of polyisobutylene and deuterated polybutadiene was measured by small-angle neutron scattering. χ was determined by fitting the scattering intensity profiles single-phase blends to the well-established random-phase approximation. Our experiments, which covered a wide range of chain lengths, suggest that χ depends on both blend composition and the ratio of the homopolymer chain lengths.