The upgrading of bitumen extracted from oil sands in Alberta, Canada is producing vast quantities of coke that must be stored in a manner that will not harm the environment. It has been assumed that these cokes are inert, and therefore should not affect any biological processes. Coke samples were incubated in two microbiological tests to determine if they are inert. One was a methanogenic bioassay, which showed that higher coke dosages led to lower methanogenic activity. In the second test, coke was incubated with heterotrophic, aerobic bacteria that are known to extract organic sulfur from coal yielding sulfate in the medium. Sulfate production was observed with one of the coke samples. Thus, the cokes are not inert. (c) 2006 Elsevier Ltd. All rights reserved.