The relative chain propagation constants and the regioselectivities of the oxidations of o‐cymene and 2‐isopropyl‐1,4‐dimethylbenzene were determined by competitive oxidations of the hydrocarbons with cumene. As expected, the reactivity of the tertiary CH bond of the isopropyl group is considerably decreased by o‐methyl groups. Also in α‐isopropylnaphthalene a considerable decrease in the reactivity of the tertiary CH bond takes place. The decrease of the chain propagation constants effects a decrease of the oxidabilities of o‐substituted isopropyl aromatics. In the case of the methyl isopropyl benzenes the increase of the chain termination constants by primary peroxy radicals must also be taken into consideration. This results in a decrease of the oxidabilities which can be observed even in p‐cymene (in comparison with cumene).