The main focus of this paper is the description of the competition between the silica-to-rubber coupling and the matrix crosslinking for the added free sulfur during vulcanization. Therefore two statistical design experiments with a variation of the amount of the sulfur - functional silanes Si 69, respectively Si 266, were performed. It was observed that on the one hand an increase of the amount of silane leads to an increase of the amount of silica-to-rubber bonds, resulting in higher moduli and a strongly reduced elongation at break. On the other hand, the silica-to-rubber coupling consumes free sulfur resulting in a lower matrix crosslinking density. Due to the simultaneity of both crosslinking reactions, the ratio of the matrix- and silica-rubber network cannot be varied independently. In order to achieve an optimum reinforcement and to fulfill the performance demands, the ratio of the silane and the sulfur has to be adjusted very precisely. The results of this study give a better insight into the reinforcement of silica-filled rubber compounds and is of value for the compounder's work to optimize the product performance.