This article reports the purification and surface modification of coal ash silica and afterwards its utilization as reinforcing filler in solution-styrene-butadiene rubber/butadiene rubber (S-SBR/BR). The coal ash silica free of unwanted metal ions/mineral oxides was obtained using phosphoric acid. The chemical composition of the purified coal ash silica in comparison to impure coal ash indicates the presence of characteristic hydroxyl functional group at 3440 cm(-1) and an increase in the oxygen content as determined with the help of Fourier Transform Infrared and x-ray photoelectron spectroscopy, respectively. The contact angle analysis shows that after purification the polar part of the surface energy increased to 17.5 from 12 m Jm(-2). While the surface area increased to an order of magnitude, i.e., 11.6 to 110.5 m(2) g(-1). The modification of purified silica particles with bis(3-triethoxysilylpropyl) tetrasulfane reveal the functionalization of hydroxyl to -Si-O-R groups as detected at 1560 cm(-1). As a consequence, the modified silica based S-SBR/BR composite resulted in improved mechanical properties due to enhanced silica-rubber interaction. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1493-1501, 2010