Diazomethane reacts with hydrogen-terminated porous or single crystal silicon surfaces under irradiation with the 365 nm line from a mercury lamp. The surface Si-H-x groups are lost and an HF-resistant, polymeric hydrocarbon species is formed. The mechanism is proposed to commence with insertion of singlet methylene into the Si-H bond as in the analogous reaction of molecular hydrosilanes. However, the infrared spectra indicate that -CH2- groups rather than terminal -CH3 are dominant on the surface after reaction. This is attributed to an oligomerization which may proceed via attack of singlet methylene on C-H bonds or via a radical process. In the case of porous silicon, significant oxidation occurs in parallel to the reaction with diazomethane due to the presence of trace water from the reagents used to generate diazomethane. The oxidized silicon species can, however, be removed selectively by washing in aqueous 48% HF without affecting the integrated intensity of the C-H stretching bands observed by FTIR spectroscopy. In combination with an analysis of the IR spectroscopic data, we conclude that the hydrocarbon species on the surface are anchored via robust Si-C bonds rather than the labile Si-O-C linkage. This is further supported by the stability of diazomethane-treated, polished single crystal wafers to in-air STM and electrochemical cycling in aqueous media. The diazomethane treatment also shifts the flatband potential in a negative direction in aqueous NaCl compared to Si(1 1 1)-H surfaces.