High-pressure methane sorption isotherms were collected on selected Paleozoic shales from the Sichuan Basin. Excess sorption measurements were performed on shales with varied water content (dry, moisture equilibrated at 33%, 53%, 7S%, and 97% relative humidities) at 39 degrees C and up to 25 MPa. Water uptake isotherms were collected at 24 C and parametrized by the Guggenheim Anderson de Boer (GAB) model. The effect of organic richness, mineral compositions, and pore structure characteristics on water uptake and methane sorption behavior has been investigated. The mechanism responsible for the decrease in methane sorption capacity of moisture-equilibrated shales is discussed. Water uptake of shales is primarily controlled by clay minerals, and shows a positive correlation with clay mineral content. Water sorption isotherms of shales can be approximately expressed as the sum of the isotherms of individual clay minerals on a mass-fraction base. Methane sorption capacity of these shales is controlled by TOC content. The maximum Langmuir sorption capacity of shales under both dry and 97% RH conditions correlates positively with TOC content. Compared to dry conditions, methane sorption capacity of shales moisture-equilibrated at 97% RH is reduced by 44% to 63%. The experimental results indicate a stepwise decline in methane sorption with increasing water content. Evolution of sorption capacity, as a function of water content can be divided into three stages: (1) initial decline stage where the decrease of methane sorption capacity is mainly due to competitive sorption of methane and water on hydrophilic clay minerals; (2) steep decline stage where clusters of water molecules block pore space and reduce the sorption capacity significantly; and (3) slow decline stage, where a contiguous water phase successively fills the macropores and slightly reduces methane sorption by volume displacement.