The addition of a coupling agent to silica-rubber compounds enhances the filler-matrix compatibility. Under certain mixing conditions the surface of the filler may be only partly activated, which may have an adverse effect on the properties in the final product. A mixing temperature of at least 130 degreesC is necessary to ensure that the reaction between the coupling agent and the silica proceeds, whereas the coupling agent starts to react with the rubber or to donate sulphur, resulting in scorching, at temperatures above 160 degreesC. No scorching is observed when a coupling agent without sulphur is used. The reaction of the coupling agent to the silica surface or the rubber matrix is activated by the zinc oxide present in the compound. Another important parameter is the mixing time. It was observed that the coupling agent must be mixed with the silica for at least 10 minutes at 150 degreesC to obtain a sufficient degree of coupling.