Self-retention Of SO2 by ash of different grades of Kolubara lignite was experimentally investigated in a laboratory furnace. The peculiarity of this type of coal is that in the open pit there are distinct layers of coal and ballast matter, which complicates the formation of representative samples. Two grades of this coal (differing in ballast matter content) were investigated using 3 sieved fractions: 1-1.6 mm, 2.5-3.15 mm, and 4.76-7 mm. It was found that particle size had no significant effect on the sulfur self-retention efficiency (etaSO(2)). The effect of ballast matter content on etaSO(2) was investigated by classifying two sieved fractions into classes with different density ranging from < 1000 kg/m(3) to > 1600 kg/m(3). It was found that most of sulfur self-retention occurs in less dense particles., For this coal only 60% of the total calcium was found to be active in relation to sulfur self-retention capability. Most of the active calcium and sulfur were found to be present in particles of lower density, which explains their dominant contribution to overall sulfur self-retention.