The vertical distributions of the important highly reactive stratospheric species, ClO and HO2, have been inferred from Spacelab 3 (May 1985) Atmospheric Trace Molecule Spectroscopy (ATMOS) measurements of more detectable radical and reservoir species. A simple steady state algebraic expression for ClO, utilizing the observed ClONO2/NO2 abundance ratio, approximates the ClO results of a time-dependent photochemical model at sunset (30¿N). Balloon measurements of ClO and comparisons of time-dependent photochemical model calculations of ClONO2 and HCl with the corresponding ATMOS profiles suggest that the actual ClO values are less than the time-dependent model profiles for ClO at sunset and sunrise (47¿S). Errors in the current model simulation of the partitioning among the principal free chlorine species (HCl, ClONO2, and ClO) are indicated. However, a comparison of model results with observations of HOCl suggests that the calculated ClO abundance is not a significant overestimate of actual values. A similar methodology is applied to inferring the HO2 distribution, either algebraically using the ATMOS-measured HO2NO2/NO2 abundance ratio or numerically utilizing the time-dependent model. The algebraic results agree with the time-dependence model values to better than a factor of 2 at sunrise. The accuracy of the time-dependent model calculations of HO2 is suggested by the agreement between model results and ATMOS observations for HO2NO2 and balloon measurements of HO2. This confirms for the first time the procedure suggested previously by a number of authors or deriving HOx abundances from observed fields of O3 and H2O. ¿1991 American Geophysical Union