Thermal isomerization of t-butylphenyl ether alone or in the presence of phenol was studied between 180C and 340C at reaction durations ranging from 0.5 to 0.7 s. The experiments proved for the first time that isomerization occurs through a homolytic process in the first step. Activation energy of the uncatalyzed process (8kcal/mol) decreases significantly (to 2 kcal/mol) for the thermal isomerization of the ether with 10% phenol added, proving the