In this study, we explore the structural, electronic and catalytic properties of bimetallic nanoparticles of the form Au25−xAgx(SR)18 (for x = 6, 7, 8). Due to the combinatorial enormity of the number of different alloyed structures, we choose 500 random configurations corresponding to each alloying level and energetically optimize their structures. Here we report the properties of the lowest energy structures and determine the most favorable Ag alloying sites for these systems. We also show that nanoalloys with one Ag at the center and the rest in the outer shell of the Au13 kernel are less energetically favorable than the ones with all the Ag atoms occupying the surface of the Au13 kernel. We further present experimental results showing that catalytic oxidation of CO is adversely affected due to Ag alloying. We provide qualitative and quantitative evidence to explain this reduction of the catalytic activity using Fukui functions and average adsorption energies respectively.