The properties of mechanically mixed suspensions of fumed silicas in polydimethylsiloxane (PDMS) were studied. The work focuses on two aspects: (i) Adsorption of PDMS onto silicas by means of the mechanical mixing of the bulk polymer and untreated or chemically modified fumed silicas. The silica-PDMS suspensions were completely dispersed or only swollen by a solvent, depending on the filler concentration, the silica surface chemistry, and the polymer molecular weight. The bound rubber content of the different preparations was determined by a weight difference technique and by carbon analysis of the samples and was compared with previously reported values in similar systems. (ii) The effect of the mechanical mixing on the final aggregate size of the silica and on the polymer molecular weight distribution. One PDMS and silicas of different surface area and surface chemistry were used. The final size of the aggregates after a long mixing time with the bulk polymer was approximately the same in all cases. Copyright 1997 Academic Press. Copyright 1997Academic Press