The scope of this article is the study of peroxide curing of two nitrile rubbers with low and high nitrile content. The peroxide efficiency can be much higher than one, and the polymer structure determines the mechanism of cross-linking. In the rubber with low nitrile content, the peroxide radical may give rise to a polymerization reaction between adjacent double bonds generating a heterogeneous network with a negative,e effect on the vulcanizate properties. On the contrary, in the nitrile rubber with high nitrile content, this negative effect it is not present or is present to a lesser extent, and their vulcanizates show good physical properties. (C) 2005 Wiley Periodicals, inc.