AbstractThe rate of the conversion of benzene into biphenyl (“tank flow” reactor, atmospheric pressure, nitrogen dilution, 540°) is found to be highly dependent on type and concentration of additives. For example, small amounts of sulfur or oxygen lead to strong retardation, but a little formaldehyde or C2Cl6has a profound accelerating effect. Wall (memory) effects are also observed.These facts point to a radical chain mechanism ‐ involving H· and phenyl radicals ‐ rather than to a (bi)molecular process as was recently proposed byFieldsandMeyerson. Other phenomena (H/D exchange, oxidation to phenol) are also consonant with a (semi)quantitative description based on thermo‐chemical kinetics.The thermolysis of other arenes can be explained analogously; thus, biphenyl (giving benzene, ter‐ and quater‐phenyls) forms H·, C6H5‐C6H4· and phenyl radicals as intermediates, the latter fragments arising through H· + biphenyl → (C6H6‐