Si-29- and C-13-CP/MAS-NMR-spectroscopy were used to investigate the influence of substance-specific parameters such as silane concentration, type of silane, pH and moisture content of the filler on the silica/organosilane reaction mechanism. After reviewing several theoretically conceivable reaction models, one horizontal reaction model was adopted which is able to describe all the test results, According to this model, the silica/organosilane reaction consists of two main reactions, The primary reaction is defined as the formation of bonds between silane and silica with loss of ethanol, This reaction continues in the form of a secondary reaction which proceeds more slowly and during which further bonds are formed between adjacent silane molecules now located on the silica surface.