AbstractQuantitative analysis of the products formed in 1,1′‐azoisobutane pyrolyses in the temperature range of 553°–602°K has shown that the major reactions of the iso‐butyl radical areAnalysis of initial rate data gave log10k4/(kc)1/2(cm−3/2.mol1/2.sec−1/2) = 7.54±0.44 − (136.5 + 4.8) kJ/mol/2.303RT, the Arrhenius parameters obtained being in good agreement with thermodynamic data for reaction (4). Measured values ofka/(kc)1/2wherekais the rate constant of the reactioniC4H9+ AIB →iC4H10+. AIB were consistent with published parameters determined by photolysis of 1,1′‐azoisobutane. Combination of photolysis and pyrolysis data gave log10ka/(kc)1/2(cm3/2.mol−1/22.sec−1/2) = 3.68 ± 0.15 − (27.2 ± 1.2) kJ/mol/2.303RT. The crosscombination ratio for methyl and iso‐butyl radicals has been found to be 0.25, indicating that the geometric mean rule does not hold f