The hydrogenation of diene-based polymers via diimide is a very attractive alternative to the conventional catalytic hydrogenation route based on gaseous molecular hydrogen. However, serious crosslinking always accompanies it and limits the end-use properties of the polymers. Gel formation in polymers that are hydrogenated via diimide has been investigated in detail through an inspection of all the chemical reactions involved in the process. The results indicate that hydrogen peroxide decomposition, some reactions related to oxygen, and the redox reaction between hydrogen peroxide and hydrazine are capable of generating radicals. However, radicals generated in the aqueous phase do not appear to initiate the crosslinking of diene-based polymers in the latex form. It is proposed that the primary radicals giving rise to crosslinking are generated in the polymer phase in situ, and the step responsible for generating these organic radicals is possibly the diimide disproportionation reaction. (c) 2005 Wiley Periodicals, Inc.