The thermal decomposition and runaway reaction of cumene hydroperoxide (CHP) in cumene were studied by thermal analysis and by using the VSP-II adiabatic calorimeter. Decomposition kinetics were determined in various CHP concentrations in cumene. The reaction order of 20wt%, 35wt%, 50wt%, 65wt%, and 80wt% was determined to be 0.5, and the Arrhenius parameters were measured as Ea (kJmol(-1)) = 122.0 +/- 3.0 and In A (min(-1) M-1/2) = 30.0 +/- 1.2. This new determination of the reaction order as equal to one half implies the same mechanism of decomposition regardless of CHP concentration. Chromatography and IR spectroscopy was combined with calorimetry data to determine the thermal decomposition kinetics. The preliminary rate determining step was considered to be the decomposition of CHP by dimer association. A reaction mechanism for CHP thermal decomposition is proposed.