Key points: • The detrimental impact of preadsorbed water on the methane adsorption capacity and rate is more pronounced than that of water vapor • The distributions of adsorbed methane and water in shale pores were compared between the SAWM and PMMS • Water vapor preferentially enters mesopores (1.5-20 nm), and preadsorbed water mainly occupies micropores (0.3-1.5 nm) Abstract Water plays an essential role in shale gas migration and adsorption, and most studies on the influence of water on shale gas adsorption refer only to moisture-equilibrated shales. To investigate the impact of water vapor on methane adsorption in shales, three experiments were conducted and compared: (1) pure methane adsorption onto dry shale (PMD), (2) pure methane adsorption onto moisture-equilibrated shale (PMMS), and (3) simultaneous adsorption of water vapor and methane (SAWM) onto shale. Comparison of the experimental results reveals that the detrimental impact of water vapor on methane adsorption is inferior to that of preadsorbed water. Two mechanisms, i.e., water blocking and adsorption competition, are responsible for the reduction and difference in the methane adsorption capacity and adsorption rate between the PMMS and SAWM. Compared to the PMD, the methane adsorption capacity decreases by 81-96% in the PMMS, and by 20-49% in the SAWM. Methane adsorption equilibrium is achieved the fastest in the PMD. Before the equilibration degree reaches 95%, methane adsorption during the SAWM progresses more rapidly, while the reverse occurs when the equilibration degree exceeds 95%. The pore size distribution and water film thickness calculations indicate that the impacts of water vapor in the SAWM on micro-to mesopores are weaker than those of preadsorbed water. In the PMMS, adsorbed water mainly