Abstract The crosslinking of EPDM has been studied by measuring the density of the solution in cyclohexane of the soluble part of samples at various extents of crosslinking. All samples had the same weight and were extracted by equal volumes of solvent. Two series of samples have been studied: chemically crosslinked (by dicumyl peroxide) and photooxidized. In the case of the chemically crosslinked samples, the variations of the density of the solution with the curing time show that the density tends towards that of the solvent, with kinetics which are a function of the peroxide content. In the case of the photooxidized samples, the results show that after a first period crosslinking is predominant leading to a decrease of the density of the solution. In a second period, chain-scissions take place and a notable increase of the density of the solution is observed.