A technique was developed to use multiwavelength aerosol extinction measurements from the Halogen Occultation Experiment (HALOE) to determine the size distribution of stratospheric H2O--H2SO4 aerosols. Although the HALOE extinction spectrum alone cannot be used to reliably infer the aerosol size distribution (except when the aerosol population contains particles larger than about 0.5 μm), the inverse problem becomes highly defined when the effective radius is known. Using theoretical relationships derived from in situ aerosol measurements, we found that the effective radius can be determined from the HALOE 2.45 μm extinction with uncertainties of about ¿15%. Using extinction ratios with the effective radii determined from the HALOE extinctions, we obtained unimodal lognormal size distributions. The HALOE size distributions are generally unbiased with respect to coincident in situ aerosol measurements. Error analysis reveals that uncertainties in the inferred surface areas and volumes are less than 30% and 15%, respectively. Inferring size distributions from the HALOE data set provides global and temporal aerosol information, which can satisfy important needs for investigations of the Earth's radiative and chemical balance. ¿ 1998 American Geophysical Union