Experiments of mixing silica, carbon black, and CaCO3 into various elastomers (EPDM, NBR, and SBR) were carried out in this paper. We have investigated the breakup of filler agglomerates in the mixing process and compared the mixing results between the internal mixer and the NEX-T continuous mixer. We have evaluated the impact of the elastomer type, filler type, filler loading, and mixing time on the breakup of filler agglomerates. When the filler agglomerates break up, the agglomerate size was reduced to a steady state size. The steady state size primarily depends on the type and primary particle size of the filler. A model was developed for the rate of agglomerate breakup. (c) 2006 Wiley Periodicals, Inc.