The effects of mixing temperature and mixing time on the reaction of 3-octanoylthio-1-propyltriethoxysilane (NXT silane) during mixing with precipitated silica and natural rubber (NR) were investigated. Results showed that the reaction between NXT silane and precipitated silica can proceed at temperatures above 130 degreesC. Because of the blocking group of NXT silane, the silane-NR coupling reactivity is low, so that the reactioii of NXT silane with NR occurs only during the curing period. There is no reaction between NXT silane and NR during mixing, which showed that the mixing time of silica-filled NR compound containing NXT silane must be longer than 10 min at 150 degreesC to obtain the desired silane coupling efficiency. With increasing mill temperature, the coupling efficiency increases. A high mixing temperature promotes improvement of silanesilica coupling efficiency, although the tensile strength, 100% modulus, 300% modulus, and hardness of NR vulcanizates decrease. (C) 2004 Wiley Periodicals, Inc.