Phase diagrams of four binary blends of polyisobutylene (component 1) and deuterated polybutadiene (component 2) were determined using small-angle neutron scattering (SANS). Our study covers N-1/N-2 values ranging from 0.23 to 0.92 and N-i ranging from about 800 to 3600 (N-i is the number of monomers per chain of component i based on a reference volume = 0.1 nm(3)). The experimentally determined binodal curves were in good agreement with the predictions based on the Flory-Huggins theory using a previously determined composition- and chain-length-dependent Flory-Huggins interaction parameter presented in Nedoma et al. et al. Macromolecules 2008, 41, 5773-5779. The experimentally determined spinodal curves, which were surprisingly close to the experimental binodal curves, deviated significantly from predictions, an observation for which we offer no quantitative explanation.