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• The micropores of fusain are extremely developed, especially the pores with D = 0.33–0.38 nm. • Aromatic interlamellar pores are important components of the fusain micropore structure. • The Langmuir volume (V L) under dry conditions is much larger than that under wet conditions. • The micropore volume and surface area of fusain did not positively correlate with the V L. • The blocking effect of water molecules resulted in the decrease of methane adsorption. Fusain is widely distributed in coal seams and studying its porosity and adsorption capabilities helps clarify the physical properties of coal reservoirs and gas adsorption mechanisms. The microporous structure characteristics and gas adsorption mechanism of fusain in lignite were studied based on methane and CO 2 adsorption experiments, combined with nuclear magnetic resonance (13NMR) and high-resolution transmission electron microscopy (HRTEM) measurements. Numerous micropores were found in the sample, and the pore volume and specific surface area were relatively large, which is similar to the results of the micropore test for meta-anthracite. The adsorption capacity of fusain was higher under dry base conditions but significantly lower under air-dry base and equilibrium water conditions. The chemical structure analysis results indicate that the micropores of fusain are mainly the interlamellar pores of the aromatic layers and the pore channels formed by them. Under dry conditions, the water in the microporous structure was removed, which provides an effective space for gas adsorption, resulting in more micropores for CO 2 adsorption and a higher methane adsorption capacity. Under wet conditions, the water occupies the adsorption site of methane, which hinders the gas entering the microporous channel of the fusain, resulting in a low adsorption capacity. On comparing the pore structure characteristics of fusain and meta-anthracite, it was found that despite possessing similar chemical structure characteristics of high polycondensation and similar micropore structure, there were clear differences in pore size, and their adsorption properties for methane were different. [ABSTRACT FROM AUTHOR]