Two sets of coal-bearing measures, the Upper Triassic Xujiahe Formation (T(3)x) and the Upper Permian Longtan Formation (P2I), have been extensively developed in the Sichuan Basin, SW China and some large gas fields related to these coal-bearing measures have been discovered. The stable carbon and hydrogen isotopic composition of alkane gases sourced from the two sets of coal-bearing measures (T3x and P2I gases) have been investigated in this study. The main controls on the gas composition and isotopic abundances of gases in the Sichuan Basin have been identified and distinguished on the basis of their molecular and isotopic composition. The carbon and hydrogen isotopic compositions of methane (delta(13)Cc1: -42.5%. to -26.8%; delta D-c1: 173%. to 107%.) vary widely and show a positive correlation with the thermal maturity of the source rocks. The delta C-13(c2) value of the T3x and P2I gases in the Sichuan Basin ranged from 28.2%. to 21.3%., and showed less variations with increasing maturity from R. values of 12% to 2.8%. However, these delta C-13.(c2) values differ greatly from that of oil-associated gas in the Sichuan Basin. Thus we provide further evidence that the value of delta C-13(c2) is a useful parameter to distinguish coal-derived and oil associated gases. The hydrocarbon gases in the Xujiahe and Jurassic reservoirs in contrast showed little isotopic difference indicating that secondary migration has little influence on their carbon and hydrogen isotope compositions. In addition, the delta C-13 values of P2I CO2 (-2.6 to 3.3%0) may reflect an origin from dissolution of the carbonates in the reservoir by acidic fluids containing H2S. (C) 2013 Elsevier B.V. All rights reserved.