A single fumed silica, consisting of spherical primary particles (— 10 nm diameter) which are fused into aggregates (—200 nm in diameter after mixing) was subjected to different surface treatments, then mixed with polymethylsiloxanes (PDMS, Mw=2000-325 000) and tested under oscillatory shear. The dynamic moduli showed a strong strain dependence, while the frequency dependence changed dramatically with the silica concentration. Experimental results are explained by agglomeration of silica aggregates through bridging polymer chains. Agglomeration increased with silica concentration, amount of silanols on the silica surface, and polymer molecular weight, and the break down of agglomeration occurred at decreasing strain levels with increasing filler concentration. Suspensions made with low molecular weight PDMS showed a different dynamic behavior due to the absence of entanglements and the small probability of bridging chains.