in addition to the previously reported 25-norhopanes and 25-norhopanoic acids, for the first time we report the identification of 25-norbenzohopanes. The hydrocarbon composition of the bitumen from Palaeozoic carbonates in northern Alberta displays molecular evidence for severe levels of biodegradation characterised by the removal Of C(30)-C(35) hopanes. Biodegradation is also indicated by the removal of C(32) and C(33) benzohopanes. The appearance Of C(31) and C(32) 25-norbenzohopanes corresponds to the decrease in C(32) and C(33) benzohopanes, suggesting that 25-norbenzohopanes originate by demethylation of benzohopane counterparts. Demethylation at C-10 in the hopanoids affects a broader class of compounds that so far includes the hopanes and hopanoic acids, as well as the benzylated hopanoid species. (C) 2009 Elsevier Ltd. All rights reserved.