The composition of the high molecular weight tail in the branched polyethylene standard reference material SRM 1476 was studied in detail using size exclusion chromatography coupled with a refractive index, a viscosity, and a light scattering on-line detector. The light scattering determinations of both molecular weight and radius of gyration point at a difference in kind between the molecular species below and above 10(6) Da. In particular, above 10(6) Da, increases in molecular weight resulted in a marked "densification" process. This is consistent with a greater concentration of long-chain branching and/or with a change in molecular architecture. Previous literature results for this polymer and a critical comparison of the performance of the different detectors are also discussed.