Selectively localizing nanofillers in a particular phase of polymer blend to achieve required properties are hardly possible. Herein, an approach to solve the above mentioned problem is reported by exploring the effect of catalyst in preferentially growing in-situ generated nanosilica particles in the phases of incompatible natural rubber (NR): carboxylated nitrile rubber (XNBR) (50:50) blend. In-situ generated nanosilica particles prepared using tetraethylorthosilane (TEOS) as silica precursor and n-butylamine (nBA) as catalyst preferentially grow in the dispersed non polar NR phase (due to the hydrophobic nature of the silica particles) and ultimately leads to significant enhancement in the blend properties. On the other hand, in-situ generated nanosilica particles prepared using TEOS as silica precursor and hydrochloric acid (HCl) as catalyst preferentially grow in the continuous polar XNBR phase (due to the hydrophilic nature of the silica particles) and ultimately does not increase the blend properties to a greater extent.