The silicon surface passivation of monolayers of organic compounds that are bound to Si surfaces by a covalent Si-C bond has been investigated. The effective lifetime tau(eff) of minority charge carriers in the surface-modified semiconductor has been determined by modulated free carrier absorption (MFCA) measurements. The results show that on 1-2 Ohm.cm p-type Si(100) surfaces modified with a monolayer obtained from CH2=CH-(CH2)(8)-C(=O)-O-CH3 maximum effective lifetimes tau(eff) greater than or equal to 130 mus can be obtained. This value corresponds to a maximum surface recombination velocity S-eff of 120 cm/s, a value that is similar to those obtained using other passivation techniques, which demonstrates that these monolayers provide an interesting alternative for silicon surface passivation. During these MFCA measurements an unusual time dependence of the effective lifetime is observed: tau(eff) rises continuously during illumination of the substrate. Kelvin probe measurements show that there is a slow shift of the Fermi level of the semiconductor under illumination, which seems to be the result of a slow, reversible filling of surface traps.