Biaryl bonds are the strongest carbon-carbon single bonds in fossil fuels, This paper examines hydrogenolysis and alkane cohydrogenolysis of biphenyl and dimethylbiphenyls, in detail, Biphenyl cleavage was found to be enhanced by copyrolysis and cohydrogenolysis with small amounts of 2,2,3,3-tetramethylbutane (hexamethylethane, HME). Much smaller enhancements were found for cohydrogenolysis with other alkanes. Increased biaryl cleavage rates, in HME cohydrogenolysis, were found to be a direct consequence of initiation by hydrogen atom generated during HME decomposition. Both biphenyl pyrolysis and hydrogenolysis mechanisms involve ipso hydrogen atom attack, followed by ejection of phenyl radical and the formation of benzene. Hydrogen atom is regenerated either through the phenylation of starting biphenyl or through the direct reaction of phenyl radical with H-2. Both propagation reactions are very fast, leading to highly efficient chain transfer. Biaryl bond hydrogenolysis was found to be first-order in biphenyl and half-order in H-2. Dimethylbiphenyls were found to undergo both demethylation and biaryl cleavage reactions during either neat hydrogenolysis or HME cohydrogenolysis. Cleavage of the much weaker aromatic methyl bond was found to be only slightly favored over biaryl cleavage in dimethylbiphenyl/HME cohydrogenolysis. The branching ratio for biaryl cleavage versus demethylation was also found to be sensitive to dimethylbiphenyl structure, The similar rates for biaryl cleavage and demethylation, as well as the structure sensitivity of the branching ratio, indicate the rate of formation and stability of the ipso hydrogen atom adduct are important in determining the rates of aromatic displacement reactions.