The photochemical reaction of iodine monochloride with bromobenzene yields chlorobenzene as enriched in either /sup 35/Cl or /sup 37/Cl depending on the excitation wavelength. A gas mixture of ICl and C/sub 6/H/sub 5/Br is irradiated by the output of a cw dye laser (01 A bandwidth) tuned to the (18,0) bandhead at 6053 A of the I/sup 37/Cl A /sup 3/II/sub 1/-X/sup 1/SIGMA/sup +/ system. At pressures of several Torr, C/sub 6/H/sub 5//sup 37/Cl is formed with a quantum yield of 10 percent and a maximum enrichment factor of 9 (75 percent /sup 37/Cl). The extent of enrichment is found to vary with starting conditions, permitting study of the mechanism for isotopically specific reaction and attendent scrambling pathways. Formation of C/sub 6/H/sub 5//sup 37/Cl involves direct reaction of I/sup 37/Cl* with C/sub 6/H/sub 5/Br to produce a vibrationally hot radical intermediate, which retains to a high degree its isotopic integrity because of more rapid elimination of Br atom than radical chain propagation. Comparison of iodobenzene with bromobenzene shows that the rate-limiting step must involve a Cl atom migration about the ring which is in competition with loss of Cl atom. While excited ICl transfers Cl to unsaturated halogenated hydrocarbons, nomore » reaction is observed with saturated halogenated hydrocarbons. The general form of this mechanism demonstrates isotopically specific synthesis involving reaction mixtures of natural abundance as starting materials.« less