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[Display omitted] • Structure and core level spectra of CoSi 2 -like to Co-rich silicides are discussed. • 200 ML Co on Si(1 1 1) forms a closed metallic Co thin film in contrast to 100 ML. • Thermostable CoSi 2 and Co-Si(1 1 1) ring clusters do not bind CO at RT. • CO adsorption at RT only occurs on Co-rich silicides and metallic Co. We report on the structure and reactivity of the Co-Si polycrystalline surfaces that form at room temperature (RT). Scanning tunneling microscopy (STM) was used to examine the surface morphology while X-ray photoelectron spectroscopy (XPS) was used to detect the chemical states of cobalt and silicon as a function of cobalt coverage. Moreover, XPS measurements after exposing the Co-Si(1 1 1) samples to CO gas provide information about CO adsorption and dissociation. When <5 ML Co is deposited on Si(1 1 1) at RT, most Co atoms diffuse into Si lattice and no CO adsorption is found at RT. This shows that coordination of Co to Si suppresses CO adsorption. Neither Co-induced Si(1 1 1)-ring cluster nor CoSi 2 crystalline film adsorb CO at RT. In contrast, CO adsorption was found at RT for cobalt doses >5 ML, an indication that Co-rich silicides and metallic Co present. The CO adsorption capacity increases with cobalt dose up to 30 ML after which it levels off, an indication that metallic cobalt sites are dominantly exposed on the surface. Furthermore, a closed metal film is formed for 200 ML dose. Some dissociation of CO was observed during heating of the CO-covered samples for Co doses ≥30 ML. [ABSTRACT FROM AUTHOR]