The network formation of silica-filled, TBBS accelerated sulfur vulcanization of cis-1,4 polyisoprene (Natsyn 2200) was studied by solid state C-13 NMR spectroscopy, and equilibrium swelling measurements. Samples with varying silica levels at different stages of cure were analyzed. It was observed that silica retards the cure reaction, and results in an overall lower cure state. Silica also has an influence on the vulcanization chemistry causing an enhancement of the cis-trans isomerization, chain scission, and the formation of monosulfidic linkages. It was also determined that the polysulfidic linkages decreased as silica level was increased. Intermolecular crosslinks decreased, while intramolecular structures (including pendant side groups and cyclic sulfur structures) increased. Cross link density, as determined by swelling measurements, decreased as the silica level was increased.