The Si(001)-2 x 1 surface saturated by water is characterized by the passivation of nearly all of the dimerized atoms by H/OH terminations, except isolated dangling bonds, whose areal density is in the range of 1.5 +/- 0.2 x 10(-2) defects per Si atom. Therefore the water-saturated Si(001)-2 x 1 surface presents similarities with the defective H-terminated Si(001)2 x I. surface (presence of monohydrides and isolated dangling bonds), on which alkene molecules are known to be grafted via a radical-based hydrosilylation mechanism initiated at silicon dangling bonds. These common features stimulated the present study devoted to the reactivity of the water-saturated surface (n(+)-doped substrate) with styrene (H2C alpha=C beta H-C6H5) at room temperature. Using synchrotron radiation X-ray photoemission spectroscopy (XPS), our aim was to estimate the extent of styrene growth and to characterize the chemistry of the adsorbed molecule. XPS showed that styrene does react with the surface: after an exposure of 6.7 L (900 s x 0.75 x 10(-8) Torr), we estimate that about 0.2 molecules per Si dimer (similar to 0.1 molecule per Si atom) are grafted on the surface. The C is XPS spectrum is consistent with a hydrosilylation product, Si-C alpha H2-C beta H2-C6H5. Indeed we found that the C Is spectral shape of styrene mono-a bonded to the water-covered surface is markedly different from that of styrene di-sigma bonded to the clean Si(001)-2 x 1 surface, confirming the specificity of the reaction product formed on the former surface. Mechanistically, a radical-based hydrosilylation reaction is the most plausible, as theoretical works indicate that the activation barrier. of the latter mechanism is much lower than that of a direct, concerted mechanism. The C is spectral shape also excludes a reaction of the molecule with surface hydroxyls, leading to the formation of monohydroxyls C beta OH(H) (radical-based reaction) or of Si-O-C linkages (Markovnikov or anti-Markovnikov addition).