The distributions of trimethyldibenzothiophenes (TMDBTs), the relationship between TMDBTs and trimethylbiphenyls (TMBPs) and the application of TMDBTs as maturity indicators have been investigated in a set of lacustrine shales from the Eocene Shahejie Formation in the Liaohe Basin, East China. The 3,5,3'-, 3,5,4'- and 3,4,4'-TMBP isomers can react to yield (2,4,6- + 2,4,8)-TMDBT, 2,4,7-TMDBT, and (3,4,7 + 2,3,7)-TMDBT by incorporating a sulfur atom into biphenyl, respectively. The absolute concentrations of (2,4,6 + 2,4,7 + 2,4,8)-TMDBT increase with those of (3,5,3' + 3,5,4')-TMBP. Similarly, the absolute concentrations of (3,4,7- + 2,3,7)-TMDBT show a similar increase with those of 3,4,4'-TMBP. The good straight line relationships (R-2 = 0.96) indicate the product-precursor links between TMDBTs and TMBPs in the sediments. On the basis of the possible differences in the thermodynamic stability among TMDBT isomers, a new maturity indicator 2,4,6-/(2,4,7 + 2,4,8)-TMDBT ratio is proposed in this study. This ratio has a good correlation with 4-/1-methyldibenzothiophen ratio (MDR) and T-max (degrees C), respectively, indicating similar behavior with increasing maturity. Both of the 2,4,6-/(2,4,7 + 2,4,8)-TMDBT ratio and MDR exhibit similar maturation trends. They gradually decrease with increasing maturity at the low maturity stages, and then are followed by a regular increase with the increasing maturity. Therefore, this ratio is an effective maturity indicator for source rocks and an oils maturity assessment (R-o > approximate to 0.55%), and it must be applied with caution as a maturity indicator for extracts of immature sediments. (C) 2012 Elsevier Ltd. All rights reserved.