A technique is presented for estimating the effective radius, surface area density, and volume density of stratospheric aerosols from infrared emission measurements. These parameters are required to assess the perturbation of the climate and chemical balance of the stratosphere following the largest volcanic eruption so far this century: that of Mount Pinatubo in the Philippines. The method uses a relationship between the surface area density and the volume density derived from balloon-borne measurements of the Mount Pinatubo aerosol cloud made at Laramie, Wyoming. It is shown that the aerosol emission value is well approximated by a linear function of effective radius and aerosol volume density. The technique relies on knowing the refractive index of the aerosol cloud, which is assumed to be composed of liquid spheres of sulphuric acid and water. It is shown that the uncertainties in the current knowledge of the refractive index of sulphuric acid solutions limit the accuracy of the inversion technique. As a case study, the aerosol effective radius, surface area density, and volume density are determined from emission measurements at 12.1 μm of the Mount Pinatubo aerosol cloud made by the improved stratospheric and mesospheric sounder (ISAMS) carried on the Upper Atmospheric Research Satellite (UARS). From these measurements it is shown that five months after the eruption the core of the Mount Pinatubo cloud had a size distribution with an effective radius of 0.5 μm, a surface area density of 35 μm2 cm-3, and a volume density of 6 μm3 cm-3. ¿ American Geophysical Union 1995