The partitioning of polymer and plasticizer into soft and hard regions for mixtures of poly-(butyral-co-vinyl alcohol) and dihexyl adipate has been determined by magic-angle-spinning13C NMR. The soft regions are detected by Fourier-transform techniques with scalar decoupling and the hard regions by cross-polarization with dipolar decoupling. This two-phase character of plasticized polybutyral is also observed in small-angle neutron scattering experiments in which integrated scattering intensity increases linearly with plasticizer concentration. We attribute the soft regions to liquid plasticizer containing mobile polymer and the hard regions to solid polymer associated with partially immobilized plasticizer. There is no chemical exchange between hard and soft regions on a 1-s time scale. The frequencies but not the amplitudes of cooperative main-chain motions of the polymer in the hard regions are influenced by interactions with the soft regions. This result is incorporated into a two-phase domain model that is used to rationalize the macroscopic stress-relaxation properties of these plasticized polymers.