The thermodynamics of binary and multicomponent mixtures comprising polymethylbutylene, polyethylbutylene, and polymethylbutylene-block-polyethylbutylene was studied by small-angle neutron scattering. The Flory-Higgins interactions parameter, deduced from these measurements, was independent of blend composition, as well as molecular weight and molecular architecture of the components, over the accessible temperature (27-201 degrees C) and pressure (0.01-0.86 kbar) range. At temperatures above 130 degrees C, the interaction parameter becomes independent of pressure. These findings are compared with previous experimental studies on the thermodynamics of polymer blends. To our knowledge, this level of agreement with the Flory-Huggins theory has not been seen in any other polymer blend.