The effect of benzene and a wide variety of substituted benzenes upon the photochlorination of 2,3-dimethylbutane (DMB) has been investigated under standard conditions (0.15 M DMB, 2.0 M arene in CCl 4 at room temperature). The standard selectivities, S std m, are given by the product ratios [2-ClDMB]/[1-ClDMB] under these conditions. For arenes which are less electron rich than benzene, log S std m values decrease monotonically with the increase in the arenes' ionization potentials and with the decrease in the arenes' π-basicities. For arenes which are more electron rich than benzene, S std m values are greater than for benzene only for the mono- and dialkylated benzenes and 1,3,5-tri-tert-butylbenzene. Chlorination in the presence of trimethylbenzenes, more highly methylated benzenes, and anisole afforded less selective mixtures of DMB chlorides than benzene. The reduced selectivities of the most electron-rich arenes is attributed to the low reactivities of their Cl •/arene π-complexes and the low yields of DMB chlorides, much of which are formed by hydrogen abstraction by the free Cl • atom. Ipso substitution occurs with bromo- and iodobenzene and these two arenes are more selective than would be predicted. It has also been demonstrated by laser flash photolysis that Cl •/arene and Br •/arene π-complexes will appear to react with O 2 only if the arene contains substituents which can donate hydrogen to the halogen atom and/or complex, viz., CH 3, C(CH 3) 3 and OCH 3 substituents for chlorine but only CH 3 and OCH 3 for bromine. This apparent reaction is due to interference by O 2 in the process which otherwise would very rapidly "regenerate" the halogen atom/arene complex following such hydrogen abstraction. Published 1989 by the American Chemical Society.