Bromobutyl elastomer (BIIR) can be used for sealing medical drug delivery devices due to its high resistance to permeability by moisture and gases which are known to deteriorate the performance of pharmaceutical drugs. However, its use is restricted since the traditional accelerators used for crosslinking BIIR leave reaction residues which can leach out and contaminate drugs. An accelerator based on xanthogen polysulphide (DIXP) does not leave reaction residues as it is totally consumed during crosslinking. From these considerations DIXP was used to formulate a BIIR composition for seals in medical devices. The isothermal kinetic reaction curve for BIIR showed that the crosslinking region consists of two parts. The first part had a reaction order (n) = 1 and the second stage had a lower n = 0.6-0.7. The rate constants and the activation energies for the two stages were determined. Increasing the time and temperature variables of crosslinking increased the dynamic elastic shear modulus (G') in BIIR according to the kinetic theory of vulcanization, however the moisture ingress showed an initial decrease followed by an inflection where increasing the variables caused increased moisture ingress. This increase is believed to be caused by the volatile gaseous byproducts produced in the crosslinking reaction of DIXP which change the microstructure of BIIR. The crosslinking conditions for minimizing moisture ingress in BIIR and achieving a high G' were identified. (C) 2007 Wiley Periodicals, Inc.