This study was conducted in an effort to assess the toxicity of fluid emanating from potential sludges produced as a result of the “hot water extraction process”; employed in extracting oil from tar sand deposits in Alberta. A further attempt was made to identify specific components and/or properties which might be responsible for any toxicity observed in the emanating sludge fluid. Fluids emanating from 14 different potential sludges resulting from various proposed treatment processes were considered. All emanating fluids were observed to be highly toxic to the green algae, Selenastrum capricornutum, with an average lethal concentration (LC‐50) of 25.8% (by volume).