Metallocene synthesised HDPE with M-w=82,000 and M-n=40,000 was modified with small amounts of 1,3-benzenedisulfonyl azide by reactive extrusion at 200 degrees C with the purpose to form long-chain branches. At the processing temperature the two azide groups decompose to nitrenes that work as cross-linkers for PE. Cross-linking occurs primarily by insertion of singlet nitrenes into CH bonds. Size exclusion chromatography revealed that the modification resulted in the formation of a long-chain branched (LCB) high molecular weight fraction. The LCB was detectable with SEC for concentrations above 100 ppm corresponding to approximately 0.03-0.04 branch points pr 10(4) carbon. No signs of the formation of low molecular species due to chain scission were observed. Dynamical mechanical analysis and shear creep test showed sign of long chain branching at concentrations down to the same limit as SEC (100 ppm). These signs were thermorheological complexity, increased zero shear viscosity, increased shear thinning and increased recovery compliance. The cross-linking efficiency of 1,3-BDSA were estimated to 40-60% from comparison of SEC data with random cross-linking theory and traditional SEC-LCB analyses. (c) 2005 Elsevier Ltd. All rights reserved.