Blends of a 100 penetration grade bitumen with four different polyethylenes having up to 40 pph or 29% of polymer by weight were prepared using a Z-blade mixer at more than 160 degrees C. The blends were studied by fluorescence optical microscopy, differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). The optical measurements indicated the presence of bitumen-rich and polymer-rich phases. The DSC showed that the melting point of the crystallites was lowered and within the polymer-rich phases, the extent of crystallinity reduced by the presence of the bitumen. Annealing made significant changes to crystallite size, suggested by the raising of the melting point of the polymer crystallites. The DMTA measurements show that these polymers lower the temperature at which the glass transition softens the material, but raises the temperature at which the blend undergoes viscous flow. When the polymer fraction is over 20% to 28% the flow starts only when the crystallites melt, so the polymer then provides an associating junction network.