In this extended Study, the effect of silica reinforcement on natural rubber (NR) and NR/styrene-butadiene rubber (SBR) compounds was studied. The silica surfaces were treated with bis(3-triethoxysilylpropyl)-tetrasulfide, which is a silane-coupling agent, to chemically bond the silica to the rubber. Optimum levels of silane were determined according to the silica loading in NR compounds. Results in NR/SBR compounded with optimum levels of silane show that the cure time decreases with increasing NR content. Also, the tensile properties of the NR/SBR blends decrease with increasing SBR content. This study further confirmed that filler materials affect the physical and mechanical properties of these blends. (C) 2010 Wiley Periodicals, Inc. Adv Polym Techn 28: 224-232, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20169