The paper gives an overview of changes in heating value of Estonian oil shale, when it is burned in fluidized bed (FB) boilers, and of the accompanying phenomena - increase in boiler efficiency, fuel flow, and decrease in harmful atmospheric emissions. In the context of boiler efficiency, the paper also tackles the issue of the extent of the actual increase in efficiency and the importance of partial decomposition of carbonate in the mineral part of oil shale, and almost complete sulphur binding in FB boilers.