The morphology, mechanical, and harder properties of composites derived from phosphonium bromide ionomer derivatives of an isobutylene-based elastomer (IIR-PPh3Br) are compared with analogous non-ionic composites. Improved adhesion between the ionomer and onium ion-exchanged montmorillonite clay (NR4+-MM) allowed standard polymer compounding techniques to yield highly reinforced elastomer nanocomposites. Similar increases in the strength of interactions between ionomer and precipitated silica resulted in reduced filler agglomeration, and higher degrees of composite reinforcement. (C) 2004 Elsevier Ltd. All rights reserved.