The vulcanization characteristics of silica-filled natural rubber (NR) were studied in the presence of silane coupling agents, 3-octanoylthio-1-propyltriethoxysilane (NXT) and bis [(3-triethoxysilylpropyl) tetra sulfide] (TESPT, or Si-69). The scorch time, cure rate index, and the rate constant of NR were measured using an Oscillating Disk Rheometer in the temperature range of 140-170degreesC. The scorch time of NR decreases with increasing TESPT and NXT concentrations, and the scorch time of NR compound with NXT is longer than that with TESPT. The apparent activation energies of vulcanization for the compounds with NXT and TESPT both decrease with increasing silane concentration. The apparent activation energy of vulcanization of NR compound with NXT is lower than that with TESPT at various concentrations. (C) 2004 Wiley Periodicals, Inc.