We used Doppler-Broadening (DB) spectroscopy to investigate the effects of filling various rubber polymers with different amounts and types of carbon blacks (CB) and of silica (Si). Carbon black has important reinforcing properties for rubbers used by the tire industry specifically, therefore understanding these properties is essential as well as critical. Silica is also being used as a rubber filler to replace or complement CB, and was also investigated. The S parameter was measured for various types of CB and of Si, as a function of the relative weight (phr) used in the samples. The behavior of the S parameter was analyzed in terms of positron annihilation and positronium formation for the various samples. Results show direct correlations between the S parameter and the various physical characteristics of CB and of Si.