We have performed a series of small angle neutron scattering (SANS) experiments on blends of linear and branched polyethylenes to investigate the level of branching that is required to phase separate the system. These results confirm that the mixtures are homogenous for all compositions when the branch content is low (i.e., <4 branches/100 backbone carbon atoms for molecular weight M(w) similar to 10(5)). However, when the branch content is higher (typically greater than or equal to 8 branches/100 backbone carbons), the blends phase separate. Segregation can also be driven by isotope effects, when the molecular weight is sufficiently high so that the product of the polymerization index and the H-1/D-2 interaction parameter (N chi(HD)) > 2. In each case, the resolution of conventional SANS pinhole cameras (Q(main) similar to 10(-3) Angstrom(-1)) is adequate to determine the state of mixing, even for systems with large domains.