The thermal decomposition rate of hydrogen peroxide (H2O2) was measured in the aqueous phase at an elevated temperature. It was shown that the reaction follows first-order kinetics, and the rate constant was determined as a function of temperature. The mechanism of the aqueous phase decomposition is discussed, particularly in relation to the activation energy of the reaction. Calculations were carried out for the reactions of O2, H2, and H2O2 in the boiling water reactor sampling line. It was shown that H2O2 disappears, and the back reaction of H2 and O2 to water proceeds in the line. This was interpreted on the basis of the radical mechanism of H2O2 decomposition.