Dynamic light scattering measurements are reported for poly(ethylene-co-20.2 mol % 1-butene) (PEB10) in dimethyl ether (DME) at 110-170 degrees C and pressures to 2500 bar. The cloud-point curve for the PEB10-DME system exhibits both low- and high-pressure upper consolute solution temperature (UCST) branches. The polymer infinite dilution translational diffusion coefficient, Do, increases with increasing temperature and decreasing pressure as expected. The observed variation of Do is inversely proportional to the solvent viscosity, indicating that the polymer coil hydrodynamic size is independent of temperature and pressure. The dynamic second virial coefficient, k(D), which represents a balance between thermodynamic interactions and hydrodynamic forces, displays values that lie within the bounds expected for Theta and good solvent conditions. While the low-pressure UCST is classical in that the polymer-solvent interactions become unfavorable upon approach to this phase boundary, the high-pressure UCST branch exhibits anomalous behavior wherein polymer-solvent interactions improve as this phase boundary is approached. Such behavior suggests that the phase separation is entropic in origin and is driven by unfavorable mixing effects.