The state of mixing in blends of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) in the liquid and solid state has been examined by small-angle neutron scattering (SANS) in conjunction with deuterium labeling. In the melt, SANS results indicate that HDPE/LDPE mixtures form a single-phase solution for all concentrations, including blends containing high volume fractions (phi > 0.5) of branched polymer, for which multiphase melts have previously(1-3) been suggested. Proper accounting for isotope effects is essential to avoid artifacts, because the H/D interaction parameter is sufficiently large (chi(HD) similar to 4 X 10(-4)) to cause phase separation in the amorphous state for molecular weights (MW) > 150 000. In the solid state, after slow cooling from the melt (similar to 0.75 degrees C/min), the HDPE/LDPE system shows extensive segregation into separate domains similar to 100-300 Angstrom, in size. Both the shape and magnitude of the absolute scattering cross section are consistent with the conclusion that the components are extensively segregated into separate lamellae. Two-peak melting curves obtained for such mixtures support the SANS interpretation, and the segregation of components in the solid state is therefore a consequence of crystallization mechanisms rather than incompatibility in the liquid state.