Volcanic ash and sulphur dioxide masses from the November 2005 eruption of Karthala volcano, Comoros Islands, are simultaneously retrieved using rapid-scan, multispectral infrared measurements from the Spin Enhanced Visible and Infrared Imager on board the Meteosat Second Generation geosynchronous satellite. Retrievals are made every 15 min, which show that the ash and SO2 separate in the vertically sheared atmosphere producing a fast moving upper level (~12 km) SO2 cloud, a slower moving middle level (~8 km) ash cloud and a boundary layer (<3 km) SO2 plume. The total mass of fine ash is 0.1 1 0.05 Tg (SiO2), the upper level SO2 mass is 0.19 1 0.01 Tg (S) while the boundary layer SO2 mass is 0.009 1 0.01 Tg (S). These are the first simultaneous satellite measurements of ash and SO2 from an erupting volcano and confirm previous ideas about vertical separation. We suggest care must be exercised when using SO2 as a tracer for volcanic ash in aviation hazard warning systems.