In this paper the transport behavior of water vapor and nitrogen in a poly(butylene terephthalate) poly (ethylene oxide) block copolymer is discussed. This polymer has a high solubility for water (300 cm(3) (STP)/cm(3) polymer at activity 0.9). A new permeation set up has been built to determine the water vapor and nitrogen transport rates from mixed gas experiments at temperatures up to 80degreesC and 80 bar. The diffusion coefficient of water decreases with activity when determined from sorption and permeation experiments. The water vapor permeability decreases and the nitrogen permeability increases with increasing temperature. This behavior causes the water nitrogen selectivity to decrease 2 orders of magnitude. Their high water transport rates as well as their high selectivity for sour gases make these materials viable potential candidates for new composite membranes.