Mercury distribution in bituminous coal or lignite-fired boilers in 5 Polish power plants was detd. in field test. The Hg emission was strongly reduced (by 37-85%) by efficient sepn. of Hg-contg. dusts. but combustion of coals with high mercury content will result in Hg concn. in the off-gases that do not meet the std. requirements according to US Environmental Protection Agency regulations for existing and newly built power plants.