Abstract Incoherent quasi-elastic neutron scattering measurements on aqueous poly(ethylene oxide) solutions show that as the concentration of water is increased to 1:1 mole ratio of water to ethylene oxide monomer units, the polymer chain mobility is not enhanced. Increased mobility is only observed when the water content is increased beyond this ratio. The activation energy for viscous flow shows a similar behaviour, it is unchanged as the system is diluted from the melt to the 1:1 solution and as more water is added it falls sharply. Similar studies on the system poly(ethylene oxide)/toluene show that chain mobility is enhanced and the activation energy for viscous flow falls continuously, at all concentrations. The difference is attributed to the formation of polymer-water hydrogen bonded complexes in aqueous solution. High resolution data for the aqueous systems suggest that the molecular dynamics obey the scattering law predicted for the Zimm model. In the melt the behaviour changes towards the limit given by the Rouse model.