The use of iodine as a photolabile passivating agent for photochemical modification of silicon surfaces is demonstrated. X-ray photoelectron spectroscopy measurements show that iodine termination using iodine dissolved in benzene leads to Si surfaces exhibiting higher iodine surface coverages and lower levels of carbon contamination than previous methods. When exposed to 514 nm light in the presence of a suitable reactive molecule, such as an organic alkene, the surface iodine is removed and the reactive molecule links to the silicon surface. The results of experiments in which the polarization and angle of the incident light were varied show that the reaction mechanism is mediated by absorption of light in the bulk Si. A much greater photoattachment efficiency is obtained on heavily doped n-type silicon than on p-type silicon. It is proposed that on n-type silicon samples the photogenerated minority carriers (holes) accumulate near the surface, making the surface more susceptible to nucleophilic attack by the alkene molecules. The use of this method for photopatterning a Si surface with specific reactive groups is demonstrated.