Highly polar rubbers interact with each other through their active functional groups via condensation or substitution reactions at high temperature. Chlorosulphonated polyethylene (CSM) rubber is a highly reactive rubber, whose reactivity is due to the -SO2Cl groups. When CSM reacts with carboxylated nitrile rubber (XNBR), a chemical reaction takes place between the two rubbers at high temperature. Dynamic mechanical analysis shows that CSM and XNBR form a homogeneous blend. Fourier transform infrared (FTIR) studies support that CSM/XNBR (50/50 w/w) is a thermally induced self-cross-linking blend, when cross-linking takes place through carboxylic groups of XNBR and -SO2Cl groups or in-situ generated allyl chloride moieties of CSM. There is a loss of some -CN groups during cross-linking; this may be due to an attack on the -CN groups by HCl (produced during heating of CSM) in the presence of inherent moisture in the polymers. Due to cross-linking, an ester or amide type of linkage is formed.