We investigated the effect of H passivation on the growth of Co on Si(1 1 1) by means of core-level photoemission spectroscopy. The Co was deposited simultaneously on both in situ prepared H/Si(1 1 1)-(1 x 1) and Si(1 1 1)-(7 x 7) surfaces. This method enabled us to perform a direct comparison of the growth mode on both surfaces: the differences in the evolution of the core-level spectra have been ascribed only to the presence of the hydrogen interlayer. The decomposition of the Si 2p core-level peaks shows that disilicide-like islands appeared on both samples at the very beginning of the growth, but a cobalt-rich phase rapidly arises. The direct comparison between the two systems clearly indicates that the cobalt-rich phase is favored on the hydrogen passivated surface. (C) 2004 Elsevier B.V. All rights reserved.