The cosmogenic radioisotopes38Cl (t½= 37.3 min.) and39Cl (t½= 55.5 min.) are formed in the stratosphere in detectable quantities, and participate in the stratospheric chlorine cycles. The predominant expected chemical forms for these radioisotopes are ClO and/or ClONO2throughout most of the stratosphere. The rates for formation of HCl are sufficiently slow that most38Cl and39Cl atoms undergo radioactive decay before ever reacting to form HCl.The measurement of the fraction of38Cl and39Cl in the forms ClO and ClONO2, with a concurrent NO2measurement, can be used to measurein situthe stratospheric formation rate for ClONO2. Such experiments could be conducted at altitudes accessible to aircraft experiment