The microstructure of crosslinked blends of PE (containing 3.4 wt. % of butene comonomer) with PEcB (containing 9.4 wt.% of butene comonomer) can roughly be deduced from the microstructure of each PE (indicating a kind of phase separation process at the scale of crystallites), even though there is an important fraction of cocrystals made of chains of both PEs. Their thermo-oxidative ageing mainly leads to chain scissions. Annealing and chemi-crystallization occur in relation to the fraction of both homopolymers. The crystallised chains are protected from chain scission, and their presence at the ageing temperature has direct consequences on the way the elastomer network, initially created by the process, is modified. Thus, when the ageing temperature does not melt the large crystallites, these crystallites protect the long chain portions between chemical crosslinks. In addition, the larger the PE quantity is, the more the crystallites gather to form large scale structure, thereby promoting the percolation of the amorphous cut chains out of this structure. The mechanical behaviour of the materials at ambient temperature or at temperatures above their melting temperature, is the result of all these mechanisms. Thus, the crosslinked blend of 50wt%. of PE and 50wt.% of PEcB may be the best compromise in terms of mechanical properties and their preservation during thermo-oxidation.