Radiative transfer calculations are performed for volcanic ash clouds assumed to consist of mixtures of ash particles, ice spheres, water droplets and ash particles coated with sulphuric acid. The results show that for nascent volcanic eruption clouds a reverse absorption effects is noticeable in the infrared window between 10 &mgr;m and 13 &mgr;m, for which absorption of infrared radiation increases with increasing wavelength. This effect may be of use in detecting hazardous volcanic clouds from space. ¿ American Geophysical Union 1989