An investigation has been carried out of variation in reactants for the metal halide-catalysed reaction of Grignard reagents with organic halides in the presence of monosubstituted benzene derivatives as diluting solvents. With bromobenzene as halide, variation in Grignard reagent, in solvent for Grignard, in metalic halide catalyst, and in potential intermediate catalysts derived from metallic halides, produced no change in the ratio of isomeric biphenyls formed except when the Grignard reagent was prepared in the absence of ethereal solvent. In this case the ratio of isomeric biphenyls was more comparable with that found when benzoyl peroxide is the source of the phenyl radical. This suggests that the reduction in ortho-substitution and concomitant increase in meta-substitution found when ethereal Grignard reagents are used, may be due to formation of complexes of phenyl radicals with solvent ether. The chlorides of cobalt, iron, and nickel are suitable catalysts, but those of copper and samarium are inactive. The evidence points to the metal derived from the catalyst as the ‘active catalyst’, rather than a metallic sub-halide or organometallic species. Use of nitrobenzene as the aromatic solvent gave azoxybenzene as a major neutral product, and hence nitrobenzene cannot be used as a solvent for competitive reactions which use this source of phenyl radicals. The results of competitive reactions between t-butylbenzene and bromobenzene and chlorobenzene support the view that phenyl radicals participate in these reactions.