In this study the network formation in a new poly(ethylene-co-1,9-decadiene) polymer was compared with that formed in an 'ordinary' low density polyethylene after being crosslinked with a peroxide. The crosslink density was determined by swelling measurements and by strain measurements at different stresses. Above the melting point, physical crosslinking points, in the form of 'trapped entanglements', constitute around 60% of the total number of crosslinking points. The chemical crosslinks, which originate from peroxide and from reacted vinyl groups, are in minority. However, although the chemical crosslinks are fewer, they are indeed very important for making the large fraction of physical crosslinks permanent, i.e. that they do not slip under load. (C) 2004 Elsevier Ltd. All rights reserved.