The Flory-Huggins interaction parameter, chi, of an off-critical poly(methylbutylene)/poly(ethylbutylene) blend was measured over a wide temperature (T) and pressure (P) range by comparing small-angle neutron scattering (SANS) profiles from single-phase systems with predictions based on the random phase approximation (RPA). We show that chi, which is a nonlinear function of 1/T at low pressures, becomes a linear function of 1/T at elevated pressures (above 2.5 kbar). The pressure dependence of the phase boundary (i.e., binodal temperature) of our blend was determined from a series of dissolution experiments wherein a phase-separated sample was heated under isobaric conditions until the SANS profiles were identical to calculations based on the RPA. The binodal, thus obtained, was in quantitative agreement with that computed using the Flory-Huggins theory. The agreement was obtained without any adjustable parameters. Binodals determined by the more traditional cloud point measurements are erroneous due to the presence of large nucleation barriers.