Water diffusion in four hard-soft block copolymer systems, based on poly(butylene terephthalate) as the hard segment and a copolymer with poly(ethylene oxide) (PEO) as the soft block, has been investigated by pulsed field gradient echo NMR. The water absorption and the water diffusion coeff. depend on the compn. of the copolymer and the amt. of water absorbed in the system. The higher the fraction PEO in the polymer, the higher the amt. of absorbed water and the faster the water diffusion. The diffusion exptl. results further indicate an interaction between water mols. and the PEO units in the soft segments. For all investigated systems it is found that water mols. reside near PEO units within a distance of less than 0.5 nm for a time of the order of or longer than 1 ns. The higher the amt. of PEO in the soft blocks, the shorter the residence time of the water mols. near the PEO units.