Samples of different ashes formed during combustion of Estonian oil shale in a circulated fluidised bed (CFB) facility (in Hans Ahlstrom Laboratory, Karhula) were analysed by different methods. Binding activity of these ashes towards SO2, and the influence of temperature and the previous grinning of samples on the binding activity were! studied with thermogravimetric equipment. The absence of sulfide sulfur in these ashes and their high SO2 binding capacity were established At 800 degrees C and after 10-minute contact, up to 37 mg of SO2 per 100 mg of sample was bound. Previous grinding of the ashes increased this value by 1.8 times, almost to the level of the SO2 binding ability of natural limestone. The SO2 binding ability of the ashes studied was approximately twice higher than that of cyclone ash formed by using the pulverized firing technology.