The independent and combined effects of long and short chain branching on the thermorheological behavior of polyethylene are described. The activation energy of linear poly(ethylenebutene) increases,vith butene content to approximately 33-34 kJ/mol and then levels off for butene contents between 7 and 25 wt %. Long chain branched homo-polyethylene is thermorheologically complex and is most sensitive to temperature at low frequency. A technique for determining the activation energy spectra of thermorheologically complex materials is proposed. Short and long branches in the same system synergistically increase the zero-shear rate activation energy.