When hydrogen peroxide is produced sonochemically in water containing dissolved oxygen gas, the rate of production of the hydrogen peroxide is appreciably greater than with just an inert gas such as argon to support cavitation. This gives rise to the question as to whether the oxygen gas in the cavitation bubbles actually participates in the chemical reaction or just modifies the thermal or possibly electrical conditions within the cavitation bubble so as to favor the peroxide formation. Isotopic techniques involving oxygen‐18 enriched gas have been used to prove that approximately 30% of the oxygen in the peroxide originates with gaseous oxygen in water saturated at 1 atmos. Through the use of nonequilibrated isotopically enriched gas, the following further information has been obtained. In hydrogen peroxide molecules for which the gas is the source of oxygen, both of the oxygen atoms come from a single gas molecule without the formal breakage of the O‐O bond. The significance of these results will be discussed in terms of proposed mechanisms for sonochemical reactions. [Research partially supported by the Office of Naval Research.]