The phbtobehavior of the geminal dihalides (diiodomethyl)cyclohexane (7), (bromoiodomethyl)cyclohexane (11), (dibromomethyi)cyclohexane (17), (diiodomethyl)cyclopentane (22), 3,3-dimethyl(diiodomethyl)cyclobutane (27), and 8,8-diiodo-2,6-dimethyl-2-octehe (31) has been studied and compared with that previously observed for diiodomethane. In all solvents the corresponding vinyl halides (iodomethylene)cyclohexane (13), (bromomethylene) cyclohexane (21), (iodomethylene)cyclopentane (23), 3,3-dimfethyl(iodomethylene)cyclobutane (28), or cis- and trons-3,7-dimethyl-l-iodo-l,6-octadiene (33) were obtained, which are thought to arise from an a-halo cationic intermediate formed via initial light-induced homolytic cleavage of the carbon-iodine bond followed by electron transfer within the resulting caged radical pair, as shown in Schemes and In the case of diiodide 31 competing intramolecular trapping of the α-iodo cation afforded in addition the cyclized isopulegyl iodide (34). In polar solvents the vinyl iodides were accompanied by the nonhalogenated products methylenecyclohexane (15), 1-methylcyclopentene (25), cyclohexene (26), 4,4-dimethylcyclopentene (29), and cis- and trans-carane (35), which are thought also to arise from the α-halo cationic intermediate. l,l-Diiodo-2,2-dimethylpropane (lb) afforded 2-methyl-2-butene (6b). Except for carane (35) from diiodide 31 there was no detectable formation of cyclopropanes. In methanol the nucleophilic substitution products (dimethoxymethyl)cyclohexane (14), (dimethoxymethyl)- cyclopentane (24), and l,l-dimethoxy-2,2-dimethylpropane (30) were obtained. It is concluded that geminal dihalides undergo predominant, if not exclusive, photoreaction via initial cleavage of a single carbon-halogen bond in analogy with monohalides and that car bene intermediates are not formed. A similar conclusion has been reached previously for diiodomethane in the photocyclopropanation of alkenes. 1983, American Chemical Society. All rights reserved.