Monitoring the core shell structure in bimetallic clusters: The case of CoAg

► We study nanometric pur cobalt clusters and mixed CoAg clusters. ► We analyze the Co 2p3/2 XPS spectra for these clusters. ► We point the effects of the Si matrix at room temperature. ► The presence of Si atoms leads to a chemical shift. ► The structure is considerably modified and ternary CoAgSi clusters are formed.