We have combined contact angle measurements, Fourier transform infrared spectroscopy (FT-IR), and atomic force microscopy (AFM) to investigate the UV photoreactivity of octadecylsiloxane (ODS) SAMs (self-assembled monolayers) in air. Contact angle measurements present evidence that ozone is not the active agent in alkylsiloxane SAM degradation under UV illumination in air. A combination of UV and oxygen is necessary for monolayer degradation to proceed. AFM measurements on submonolayer coverage SAMs provide direct evidence of the photodegradation of ODS SAMs and reveal the role of defects in the degradation process. FT-IR and AFM results suggest that the hydrocarbon chain is the reactive site of the monolayers. A microscopic mechanism of the photoreactivity involving hydrogen abstraction is suggested based on the mechanism of gas-phase oxidation of alkanes. Our results have implications in clarifying the contribution from the reactivity of alkyl chains during the photomodification of SAMs. This clarification may provide insight into optimization of the SAM photopatterning processes.