Carbon dioxide (CO 2 ) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH 4 ) and CO 2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH 4 and CO 2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH 4 and CO 2 on coals. Next, the adsorption mechanisms of high-pressure CH 4 and CO 2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH 4 is less than that of CO 2 , while its reverse pressure is greater than that of CO 2 . The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH 4 excess adsorption is due to the saturation near the pore wall, while the CO 2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO 2 -enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.