Binary mixtures of normal (protonated) and perdeuteriated poly(vinylethylene), and of poly(ethylethylene), containing 50% by volume deuteriated polymer, have been examined by small-angle neutron scattering (SANS). Evaluation of SANS data obtained between 26 and 100 °C, using the RPA theory for binary polymer mixtures, reveals that both isotopic systems are characterized by a small positive Flory-Huggins segment-segment interaction parameter, 5X10-4 < x < 10-3, and an upper critical solution temperature (UCST). This isotope effect primarily derives from the reduction in carbon-hydrogen bond length that results from substituting deuterium forhydrogen in nonpolar organic liquids; the shorter carbon deuterium bonds are directly manifested as a smaller segment volume, (VH-VD)/V = 0.0019 ± 0.0004 and 0.0039 ± 0.0004 for poly(vinylethylene) and poly(ethylethylene), respectively. A prediction for the excess free energy of mixing for isotopic polymer mixtures,based on the measured differences in segment volumes and the associated differences in segment polarizabilities, accounts for the experimental findings. Two other published theories, based solely on the segment volume isotope effect, are also discussed. We conclude that nonideal mixing is a universal characteristic of deuteriated and protonated polymers.