Nuclear magnetic resonance (NMR) techniques have often been used in studies of the interaction of water molecules with complex macromolecular structures such as proteins and polymers. We have measured the diffusion coefficient of protons in water/poly (ethylene oxide) solutions as a function of concentration using a pulsed field gradient NMR technique, for values of the concentration of poly-ox in H2O from to 1.2 gm poly-ox/gm. Near zero poly-ox concentration, the slope of the D vs. concentration curve is quite large. As the concentration increases, the slope decreases, and at 1.2 gm poly-ox/gm H2O the diffusion coefficient is reduced by a factor of about 6 from the value at zero concentration. This paper presents the theory of field gradient methods used to measure diffusion coefficients, describes the apparatus and technique used for our measurements, and attempts to interpret the measured diffusion coefficients by considering a two phase model which treats obstruction and hydration effects in a unified way.