Experimental data were obtained on the concentration dependence of the fraction of total filler-bound polymer, fraction of polymer in a coherent polymer-filler gel, and fraction of solvent-dispersed filler particles in compounds of fume silica (Aerosil OX50) with natural rubber (NR) and styrene butadiene rubber (SBR). Predictions of the polymer-filler gel formation theory were tested and a good formal description of the data was obtained. The two adjustable parameters of the theory assume reasonable values. While A0 is a characteristic of the filler interaction with polymer and has similar values in NR and SBR, the functionality f of the theoretically assumed filler particle acting as a polyfunctional crosslinker of polymer chains seems to depend on the polymer as well. Dimensions of the theoretical filler particle calculated from a comparison of experiment with theory are of the same order of magnitude but smaller than those of primary filler particles.