The present invention provides a process for preparing a filled halobutyl elastomer, which includes mixing a halobutyl elastomer with at least one mineral filler and at least one phosphine modifier and optionally curing the filled elastomer with sulfur or other curative systems. Filled halobutyl elastomers prepared according to the present invention possess improved levels of filler dispersion which results in a reduction in the hardness of the compound and an increased tensile strength.