The major eruption of Mt. Pinatubo (15.1 ¿N, 120.4 ¿E) on June 15th 1991 ejected 20 megatons of sulphur dioxide into the stratosphere at altitudes of 20--30 km. The sulphur dioxide converts to sulphuric acid vapour in 30--40 days, the vapour thus formed nucleates, condensing with water to form sulphuric acid aerosol, the resident time of this aerosol is several years and significantly increases the planetary albedo. A new method is presented for the detection of sulphuric acid aerosol utilising observations made by polar orbiting satellites of brightness temperature differences at two thermal wavelengths. Time series of this difference shows an increase of 2.4 K after the eruption, a value in agreement with radiative transfer calculations based on in--situ observations of the sulphuric acid aerosol. This new method can be used to globally map the distribution of the sulphuric acid aerosol and obtain an estimate for the mass loading, necessary information for climate models attempting to study the impact of the Pinatubo eruption. ¿ American Geophysical Union 1993