A photochemical study was carried out with 3-substituted 3-chlorodiazirines with 4-biphenyl- (4a), (4-biphenyl)methyl-(4b), 2-(4-biphenyl)ethyl- (4c), and 1,1-dimethyl-1-(4-biphenyl)methyl (4d) substituents. The chlorodiazirines were prepared from the corresponding amidines (7) by a procedure involving oxidation with tert-butyl hypochlorite under phase-transfer catalysis. The crystalline nature of 4a-d was established by differential scanning calorimetric analysis, which revealed melting endotherms prior to thermal decomposition. Photochemical results in crystalline solids were analogous to those observed in solution, and the products were analyzed in terms of the corresponding singlet-state chlorocarbene intermediates (5a-d). Irradiation of 4a in solution and in crystals resulted in formation of azine 9a by reaction of carbene 5a with its precursor. Equally selective, diazirine 4d gave alkene 6d as the only product by a 1,2-Ph migration from carbene 5d. In contrast, irradiation of compounds 4b and 4c resulted in formation of two alkenes by 1,2-H shifts and formation of azines by reactions of the carbenes with their precursors. The low selectivity of 4b was rationalized in terms of structural data from single-crystal X-ray diffraction analysis, which revealed two disordered diazirine conformers and close Cl...N contacts between adjacent molecules. Rapid conformational equilibration in the solid state was also suggested by solid-state C-13 CPMAS NMR. Similar structural effects are also postulated to account for the solid-state reactivity of 4c.