A total of six giant gasfields each with proven reserves of more than 100 billion cubic metres are present in the Ordos and Tarim Basins, China. The gasfields are Sulige, Yulin, Wushenqi, Da'niudi and Jingbian in the Ordos Basin, and Kela 2 in the Tarim Basin. In this paper, we report on the geochemical characteristics of the natural gas at these fields based on studies of gas composition (150 samples), together with stable isotope data (143 samples) and helium isotope data (21 samples). Results show that the gases have high contents of C1-4 alkanes (generally over 90%) and minor contents of CO2 (below 3%). The generally high delta C-13 values of C-1-C-4 hydrocarbons indicates a significant contribution from humic source rocks such as Permian-Carboniferous and Mid-Lower Jurassic coal measures. delta(13) C-1, delta C-13(2), delta C-13(3) and delta C-13(4) values for gases in Upper Palaeozoic reservoirs were -35 to -32 parts per thousand, -28 to -24 parts per thousand, -27 to -24 parts per thousand, and -23.5 to -22 parts per thousand, respectively. The delta C-13(iC4) value is higher than that of the delta C-13(nC4 center dot) He-3/He-4 ratios vary from 10(-7) to 10(-8), indicating that the helium is of crustal (as opposed to mantle) origin. The CH4/ He-3 ratio is 10(10) - 10(11), indicating that the CH4 is of biogenic origin. None of the data is consistent with an abiogenic origin for gas in the Ordos Basin.