Poly(ethylene-co-1-butene) diffusion in ethane, propane, butane, and pentane indicates that ethane solutions are remarkably different. For propane, butane, and pentane the dynamic second virial coefficient, k(D), is negative near the phase boundary, increases rapidly to positive values within similar to100 to 200 bar of the respective phase boundary, and approaches a constant value at high pressures. For ethane, kD is also negative near the phase boundary; it increases A a slower rate and becomes only slightly greater than zero. kD for all four solvents superpose when plotted against "methyl" molar density, suggesting thermodynamic interactions rather than hydrodynamic forces dominate. For propane, butane, and pentane the hydrodynamic radius, RH, also starts at a low value near the phase boundary and increases to the same value for all three alkanes. For ethane R-H is similar to30% smaller than those for the higher alkanes, reflecting the poor quality of ethane even 1000 bar above the phase boundary.