We report the photoreactivity of H-Si(111) with dioxygen and terminally unsaturated hydrocarbons. Illumination of H-Si(111) in air with ultraviolet light of wavelength of 350 nm or shorter produces oxidized silicon; longer wavelengths cause no oxidation. When H-Si(111) is immersed in unsaturated hydrocarbons (1-octene, 1-octadecene, 1-octyne, styrene, and phenylacetylene) that have been deoxygenated, illumination with a Hg lamp results in densely packed hydrocarbon films of molecular thickness. Previous work and the results presented here suggest that the H-Si bond adds across the unsaturated bond in these reactions similar to hydrosilylation reactions known for small-molecule chemistry and produce adsorbates covalently bonded to the surface. We present spectroscopic evidence showing films resulting from terminal acetylenes consist of adsorbates linked to the surface by a vinyl group. We propose that all of these reactions occur through a radical-chain mechanism initiated by the wavelength-dependent photodesorption of surface hydrogen by UV light. We also present a method in the Appendix to quantify surface adsorbate coverage using XPS and ellipsometric data.