Realistic models of graphene oxide membrane were developed and validated to interpret the exceptional water permeation in association with XPS, TG-DSC and DVS measurements. With respect of the GO oxidization level, surface distributions of functionality agglomerates were analyzed in line with TEM observations, and 3 types of interlayer domains in slit pores of GO membranes were identified. It was found H₂O occupancy strictly follows the hydrophilicity degrees of as-defined domains upon GO hydration. Calculated sorption enthalpies and isotherms are in good agreement with experimental data, and the results indicate the dominant role of Coulomb interaction. GO expansion shows a transition from the interstratification of H₂O monolayer to the accumulation of H₂O multilayers at the interlayer distance of 0.8 nm. The evolution of both hydrogen bonds and H2O diffusivities suggests the existence of 3 types of H₂O species with different binding states and molecular mobilities. The computed H₂O permeability on the basis of sorption-diffusion theory supports the exceptional H₂O transport capacity in GO membrane.