In thermal power plants, many operational problems are caused by the use of fuels rich in calcium oxide, These problems include slagging, fouling and corrosive-erosive wear of boiler heat-transfer surfaces. Part of the sulphur dioxide produced is absorbed in the calcium oxide, in the boiler flue-gas pass. The conventional technique of firing pulverised fuel (PF) does not, however, provide effective means of absorbing the large amounts of sulphur dioxide into the ash, regardless of the amount of calcium oxide in the ash, In fluidised-bed combustors (FBCs) most of the sulphur may be absorbed by the high calcium oxide content of the ash, without the application of any separate flue-gas treatment or additional use of sulphur-oxide absorbents. This research work examined the behaviour of Estonian oil-shale ash with a high content of calcium oxide in typical FBC conditions, and focused on bed agglomeration and fireside deposits. Attention was paid to the effect of different gas atmospheres and temperatures on the sintering of the ash. Ashes of three kinds were tested: a) collected from the cyclone of a PF boiler burning oil shale, b) collected from the electrostatic precipitator of the same boiler, and c) prepared in standard laboratory procedures. They were assessed for compression strength in a laboratory-scale sintering test. Cylindrical pellets were made from the ash and exposed for four hours in a tube furnace to various atmospheres and various temperatures in the range 300-1050 degrees C. After that the pellets were tested for compression strength, and the pelletising pressure was taken as an indicator of the degree of sintering. These compression tests were complemented by chemical analyses. Changes in the mass and dimensions of the pellets were also recorded. In some experiments the pelletising pressure was varied to simulate the impact energy of different particles, in order to investigate the build-up of ash deposits on heat-transfer surfaces. Significant sintering occurred in many of the conditions tested, and the degree of sintering was found to depend on both heat-treatment temperature and gas atmosphere. In some cases higher pelletising pressure led to increased sintering. The results indicated that in a full-scale FB combustor, bed sintering would probably not be a problem, but fouling of the heat-transfer surfaces after the cyclone of the FBC could be a problem, especially with flue-gas temperatures in the range 600-900 degrees C.