A general method for the synthesis of nano-alloy dispersions of noble metals like Au and Ag in organically modified aminosilicate sol is presented with an emphasis on the influence of pH on the sol stability. The alloys of Au-Ag were synthesized by co-reduction of solutions of gold and silver salts using borohydride as the reducing agent at a suitable pH in the medium containing 3-aminopropyltrimethoxysilane (APS) stabilizer. Organosilanes with amine functional group have multiple roles in the formation of stable sols such as the formation of siloxane network structures by hydrolysis and condensation and the interaction of amino group with metal ions in solution electrostatically. Mono metal and alloy sols were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy. The stable alloy sols were found to catalyze the chemical reduction of 4-nitrophenol to 4-aminophenol and the catalytic rate constants were evaluated from UV-vis spectroscopy. (C) 2011 Elsevier B.V. All rights reserved.