1. By separative calorimetry we confirmed that isobutane oxidation in the presence of HBr has two macroscopic stages that are separate in time. 2. It was established that the first stage, which is the formation of tert-butyl hydroperoxide, is homogeneous, while the second stage, which is decomposition of tert-butyl hydroperoxide and its reaction with isobutane, is heterogeneous.