Unusually abundant long-chain monomethylalkanes (MMAs) (C25-C36) and their short-chain homologues (C14-C22) have been tentatively identified in a Late Triassic oil sand sample from the Lunnan oilfield, Tarim Basin, NW China. Molecular distributions and stable carbon isotopic compositions of these compounds have been determined to investigate their bio-and/or geosynthetic sources. The results show that the most abundant isomers of short-chain MMAs were 4-methyl (m/z 70), 5-methyl (m/z 84), 6-methyl (m/z 98) and 7-methyl (m/z 112) alkanes, whereas in longchain MMAs, there is a clear predominance of the 9-methyl and 10-methyl isomers. Combined with the compoundspecific stable carbon isotopic analysis, it is concluded that short-chain MMAs and long-chain MMAs must have different sources. The similar isotopic compositions of MMAs and n-alkanes in this sample suggest that they may share the same sources. The short-chain MMAs, as previously suggested, are probably related to a cyanobacterial origin, while the long-chain MMAs may be mainly associated with microorganisms or heterotrophic bacteria, rather than cyanobacteria.