Small-angle neutron scattering (SANS) experiments have indicated that mixtures of linear thigh density) and long chain branched (low density) polyethylenes (HDPE/LDPE) form a one-phase mixture in the melt. However, the maximum spatial resolution of pinhole SANS cameras is similar to 10(3) Angstrom, and it has been suggested that such experiments do not provide unambiguous evidence for a homogeneous melt. Thus, the SANS data might also be interpreted as arising from a biphasic melt with a very large particle size (similar to 3 mu m), because most of the scattering from the different phases mould not be resolved. We have addressed this hypothesis by means of ultra-small-angle neutron scattering (USANS) experiments, using a newly developed Bonse-Hart USANS facility, which can resolve particle dimensions up to 30 mu m. The experiments confirm that HDPE/LDPE blends are homogeneous in the melt on length scales probed by pinhole SANS and also by USANS. We have also studied blends of linear and short-chain branched polyethylenes, which phase separate when the branch content is sufficiently high. It is shown that USANS can directly resolve both the size of the dispersed phase (similar to 4 mu m) and the forward cross section [d Sigma/d Omega(0) similar to 10(8) cm(-1)], which is 6 orders of magnitude higher than for homogeneous blends.