Complete sorption isotherm characteristics of methane and CO(2) were studied on fourteen sub-bituminous to high-volatile bituminous Indian Gondwana coals. The mean vitrinite reflectance values of the coal samples are within the range of 0.64% to 1.30% with varying maceral composition. All isotherms were conducted at 30 degrees C on dry, powdered coal samples up to a maximum experimental pressure of similar to 7.8 MPa and 5.8 MPa for methane and CO(2), respectively. The nature of the isotherms varied widely within the experimental pressure range with some of the samples remained under-saturated while the others attained saturation. The CO(2) to methane adsorption ratios decreased with the increase in experimental pressure and the overall variation was between 4:1 and 1.5:1 for most of the coals. For both methane and CO(2), the lower-ranked coal samples generally exhibited higher sorption affinity compared to the higher-ranked coals. However, sorption capacity indicates a U-shaped trend with rank Significant hysteresis was observed between the ad/desorption isotherms for CO(2). However, with methane, hysteresis was either absent or insignificant. It was also observed that the coal maceral compositions had a significant impact on the sorption capacities for both methane and CO(2). Coals with higher vitrinite contents showed higher capacities while internite content indicated a negative impact on the sorption capacity. (C) 2010 Elsevier B.V. All rights reserved.