The thermal, vapour-phase chlorination of benzene derivatives above ca. 300° can be greatly enhanced by u.v. light. A number of examples (105–245°) are given. The main features, relative rates, and isomer patterns remain unchanged. Apparently, the u.v. light is onlyoperative in dissociating chlorine molecules, product-forming steps being the same in the two systems. In the photolytic process substantial proportions of highly chlorinated biaryls are also produced. These products are rationalized as being formed via dimerisation of cyclohexadienyl radicals produced by reversible addition of Cl· to arene. This interpretation is corroborated by a thermochemical–kinetic analysis. Performing the chlorination by illuminating the vapours of a refluxing arene, with recycling, monochlorinated products are obtained in high yields, rendering this simple technique the preferred synthetic method for a number of cases.