[en] The paper reports operando study of ultra-thin (50 nm) graphene oxide membranes by grazing incidence x-ray scattering in air dehumidification experiments. Absorption of water vapors in GO layers follows a modified Kelvin equation revealing condensation in an elastic slit, while desorption of water is limited by a few outer GO layers providing bottleneck restrictions to water transport and resulting in classical H2-type isotherms. GO interlayer distances (d) vary in range from 7.2 Å to 11.5 Å depending on partial water pressures in a feed stream and permeate. The permeance of water vapor through GO decreases steadily with decreasing interlayer distance between GO sheets from ∼80 000 l/(m² · atm · h) to ∼30 000 l/(m² · atm · h) falling down to negligible values below d  ≈  9.2 Å. Water transport in GO is described by Poiseuille equation and hopping diffusion depending on the number of water layers between GO planes, and was modelled with semi-empirical methods. It is shown the performance of thin GO membranes is strongly governed by the interstitial water quantity as dictated by water partial pressure. (paper)