This paper studies the mass transport properties for water vapor and nitrogen for a series of poly(ethylene oxide) (PEO) poly(butylene terephthalate) (PBT) multi-block copolymers via: (a) the permeation of a water vapor/N2 mixture (b) the sorption of water vapor, (c) the diffusion of water vapor, (d) Zimm and Lundberg cluster analysis, and (e) the state of water in these polymers by FTIR-ATR analysis. The studied block copolymers consist of a PEO segment length of 1000 g/mol containing various amounts of PBT. The water vapor permeability depends strongly on the polymeric structure and increases with the amount of PEO due to a higher solubility and diffusivity. The selectivity of water vapor/N2 increases with an increase of the amount of PBT in the block copolymer due to a relatively higher water vapor permeability compared to N2. This stems from an easier permeation of the smaller water molecule through the mixed PEO¿PBT interphase. Although the average diffusion coefficient decreases with increasing feed water activity, the mobility of water vapor increases significantly with an increase of the amount of water vapor sorbed in the polymer. This is related to the plasticization of the PEO phase in the block copolymer. The increase of the mobility with the amount of water vapor sorbed in the material is lower for block copolymers containing larger amounts of PBT, suggesting a restriction of the swelling of the PEO by the PBT phase. Water starts to cluster at higher concentrations of water in the polymer, as revealed by a Zimm and Lundberg cluster analysis of the sorption isotherm and by FTIR-ATR. The latter technique shows that the interactions between water and the polymer become weaker as the amount of water in the block copolymer increases. This observation suggests that the decrease in interaction energy between water and the polymer must be compensated by an entropy gain, because the solubility of water vapor increases with an increase of activity.