The present work focuses on the environmental effect of oil shale retorting residues originating from gaseous (GHC) and solid heat carrier (SHC) technologies. Samples were taken from Viru Keemia Grupp Oil factory (VKG Oil Ltd, Estonia). The water extracts of semi-coke samples from GHC retorting plants were non-toxic. The leachate water of SHC retorting ash was acutely toxic to Daphnia magna among tested organisms. The water extracts of semi-coke and retorting ash were acutely toxic at lower pH values and the release of anions and molybdenum increased significantly. The results show strong positive correlation between the toxicity and salinity/electric conductivity of the leachates. Release of anions in turn is related to the degree of decomposition of mineral phases during the retorting process. (C) 2014 Elsevier B.V. All rights reserved.