To estimate the environmental properties of oil shale ash-based mortars the leaching of harmful components was studied. The leachates were highly alkaline. The predominant ions were Ca2+ K+, Na+, SO42-. The leachable content of soluble components in PF ash mortars was higher in comparison with that in CFB ash mortars. Results indicated that over curing time the fraction of readily soluble inorganic components decreased and the mobility of potentially hazardous Cd and Zn did not increase. Addition of bypass dust could affect the content of leachable ions. Results give new knowledge about the environmental properties of oil shale ash-based materials, including backfilling composites for underground mining technology