Hydrogen-terminated Si(lll) reacts thermally at moderate temperatures with alcohols (RCH2OH) and aldehydes (RCHO) to form the corresponding Si-OCH2R films. The films are characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). While FTIR and XP spectroscopies suggest that films of similar composition are formed, AFM and the relative chemical stabilities of the organic films show that the two reactions do not result in structurally identical films. A mechanism for the reaction of the aldehyde with Si(111)-H is proposed which is analogous to the well-known hydrosilylation of aldehydes. The reaction proceeds either by nucleophilic addition/hydride transfer or by a radical chain mechanism via adventitious radical initiation. The alcohol reaction is similar to the chemical etching of Si(lll)-H by water and short-chain alcohols. This reaction proceeds by nucleophilic attack followed by loss of dihydrogen. Traces of ammonium fluoride or water on the surface result in etching of the terraces on a time scale which is much faster than the reaction of the alcohol but not of the aldehyde. This etching can be completely suppressed by the addition of chlorotrimethylsilane to the reaction mixture. This reagent quickly scavenges both water and fluoride from the surface and reaction mixture. It is suggested that this may be a useful reagent to scavenge undesirable nucleophiles during wet chemical modification of Si(111)-H.