An unusual cross-linking of brominated poly(isobutylene-co-isoprene), BUR, has been shown to occur in the presence of the normally nonnucleophilic amidine bases 1,8-diazabicyclo[5.4.0]undec-7ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN). Model compound studies using brominated 2,2,4,8,8-pentamethyl-4-nonene (BPMN) have provided structural information which indicates that this cross-linking most likely occurs through a sequence of reactions which involves the following: (1) N-alkylation of the amidine base to an amidinium salt, (2) hydrolysis of the salt to an ammonium lactam, (3) deprotonation of the ammonium lactam to an amino-lactam, and (4) a second amine N-alkylation. The ability of DBU and DBN to undergo N-alkylation with BIIR has been confirmed by reaction of BIIR in solution with an excess of each of these bases to give N-alkylation products that have been characterized spectroscopically.