Utilization of Estonian oil shale in thermal power plants causes many operational problems. These problems include slagging, fouling and corrosive-erosive wear of steam boiler heat transfer surfaces. Part of sulphur dioxide produced is absorbed in calcium oxide in the boiler flue gas ducts. The conventional technique of pulverized firing (PF), however, does not provide absorption of extremely high amounts of formed sulphur dioxide, regardless of large amounts of calcium oxide in the fuel ash. In fluidized bed combustors (FBC), on the other hand, most of the sulphur may be absorbed by the ash of high calcium oxide content, without application of any separate ale gas treatment system or additional use of sulphur oxide absorbents. This research work deals with the sintering behaviour of Estonian oil shale ashes. The effect of different gas atmospheres and temperatures on the ash sample sintering was paid special attention to in the course of this study. The different types of ashes tested included: (i) ash collected from the cyclone of a PF oil shale boiler, (ii) ash collected from the electrostatic precipitator of the same boiler, and (iii) oil shale ash prepared by standard laboratory procedures. The ashes were tested for their compression strength after a sintering test under laboratory conditions. First, cylindrical pellets were made from the ash samples. Then the pellets were exposed to various atmospheres, and various temperatures in the range of 300-1050 degrees C, in a tube furnace for four hours. After the exposure, the pellets were tested for compression strength, and the pellet-crushing stress was taken as an indicator for the degree of sintering. The compression tests were complemented with chemical analyses of heat-treated ash pellets. Changes of mass find dimensions of the heat-treated pellets were recorded as well. Some experiments were also made to simulate the impact energy of different ash particles, and to investigate the buildup of ash deposits on the heat transfer surfaces of boilers. This was done by varying the pellet-making pressure. The tests showed that significant sintering occurred under most of the conditions tested. The sintering of oil shale ash was found to depend on both heat-treatment temperature and gas atmosphere. Higher pressure at pellet-making resulted in some cases in increased sintering.