The non-catalysed reaction of chlorine with biphenylene yields 2-chlorobiphenylene as the main product, and tetra-chloro-, and trichlorohydroxy-tetrahydrobiphenylenes. Dechlorination of the tetrachloride and hydroxychloride gives biphenylene. The reaction of biphenylene with sulphuryl chloride also gives 2-chlorobiphenylene and tetrachlorotetrahydro-biphenylene in very poor yield. An attempt to chlorinate biphenylene with phosphorus pentachloride was unsuccessful. Methoxybiphenylene has been made by combined hydrolysis and methylation of 2-benzoyloxybiphenylene. Mono-substitution occurs at the 3-position on chlorination of 2-methoxy-biphenylene with molecular chlorine, giving 2-methoxy-3-chlorobiphenylene, but no chlorination occurs with sulphuryl chloride, either on standing or on reflux. Direct chlorination of 2-nitrobiphenylene gives 2-nitro-6-chlorobiphenylene in poor yield, but nitration of 2-chlorobiphenylene gave a comjilex mixture. Attempts to chlorinate 2 ,6-dimethoxybipheiiyleiie, 2-chlorobiphenylene and 2-bromobiphenylene did not give identified substitution products, but chlorination of 2-chlorobiphenvlene with large excess of chlorine in acetic acid gave a tetrachlorobiphenylene. Direct chlorination of both 2-benzoyl- and 2-acetyl-biphenylenes, and chlorination after sulpl.onation, gave an inseparable mixture of mono- and dichloro-derivatives. 2-Benzoyl-7-chloro- and 2-acety 1-6-clilorobiphenylones were made by Friodel-Crafts benzoylation and acetylatiou respectively, of 2-chlorobiphenylene-Chlorobiphenylene has been made by pyrolysis of 2- and 3-chlorobiphenylone iodonium iodide with cuprous oxide in poor yield. Finally, pyrolysis of -4-chlorobiphenylene iodonium iodide and 2,4-dichlorobiphenylene iodonium iodide with cuprous oxide gave 1-chlorobiphenylene and 1,3-dichloro-biphenylene respectively. The comparative study of the N.M.R., U.V. and I.R. spectra of the chlorobiphenyleneSand those of their bromo-analogues was extensively and successfully used to distinguish between structural isomers.