The kinetics of gas-phase thermal transformations of 4-methyl-, 4,4′-dimethyl-, 4-tert-butyl-, and 4,4′-di-tert-butylbiphenyls was studied over the temperature ranges 813–848, 733–793, 703–763, and 703–763 K, respectively. Competition between cracking and alkyl substituent isomerization at aromatic nuclei of the initial substance and products formed was observed for tert-butylbiphenyls. The kinetic parameters of the overall transformation of alkylbiphenyls (ABPs) were determined. Recommendations for the determination of the critical temperatures of ABPs, their synthesis, processing, and use of articles with structural fragments in molecules identical to those studied in this work were given. It was found experimentally that 2-, 3-, and 4-methylbiphenyls and their mixtures with toluene were thermally unstable over the whole composition range at critical temperatures (600–780 K).