The silica-silane system is used in modern tire tread compounds to improve wet grip and rolling resistance. An efficient silica / silane coupling during the mixing process is necessary to achieve the required in-rubber-properties. In order to understand this coupling reaction in detail, the silanol group density of the silica was characterized in depth. Furthermore, the reaction of a mercapto silane (Si 263 (R)) with two different silicas was studied with special infrared (IR) techniques. The infrared spectrum of the v (SiOH) region shows the behavior of SiOH groups during the reaction and contributes to clarify their role in the silane grafting reaction.