The conformations of molecules in a diene (butadiene, isoprene) complex with hydrogen peroxide and isopropyl alcohol, for which local energy minima are identified, and a mechanism of hydrogen peroxide decomposition have been studied by the quantum-chemical density functional theory method. It has been shown that depending on the initial multiplicity of the system, the decomposition can result in the formation of different intermediate and final products, which indicates the probability of singlet to triplet conversion of hydrogen peroxide in the real system.