The photochemical theory of the partitioning between the chlorine species ClONO2 and ClO in the lower and mid stratosphere is tested for the first time using simultaneous measurements of ClO, NO2, and ClONO2. The theory suggests that over most of the day the production of ClONO2 through the three-body reaction of CIO and NO2 is balanced by the loss of ClONO2 through photolysis. Our analysis tests this theory by utilizing simultaneous measurements of these species from instruments aboard the Upper Atmosphere Research Satellite (UARS), as well as calculations of the photolysis rate of ClONO2 and laboratory measurements of the rate constant for the reaction between ClO and NO2. Between 32 and 10 hPa (~24 and 32 km), averaged instantaneous production and loss rates of ClONO2 agree within ~10%. At 46 hPa (~21 km), the agreement between averaged instantaneous production and loss of ClONO2 is poorer than at higher altitudes, but still within uncertainties. This analysis provides no evidence for any problems in our understanding of the partitioning between the chlorine species ClONO2 and ClO over the range of pressures (46 to 10 hPa) and latitudes (60 ¿S to 60 ¿N) considered here. ¿ American Geophysical Union 1996