The poly(ethylene oxide) (PEO)/water system is investigated using small-angle neutronscattering (SANS). This system associates to form hydrogen-bonded clusters at high enough concentrations.Two correlation lengths are observed: one long range representing cluster sizes and the other short rangerepresenting polymer chain correlations. Clusters are formed at a volume fraction of 4% hPEO in D2O.An LCST transition is obtained between a mixed phase (through hydrogen bonding) and a demixed two-phase region. Solvent deuteration is seen to enhance hydrogen bonding. Deuteration of the polymerbackbone is seen to enhance hydrophobic interactions. The average polymer contrast match method failsdue to the isotopic dependence of specific interactions. Pressure was seen to lower the LCST by breakinghydrogen bonds. At even higher temperature (beyond the boiling point of water) a UCST transition wasobserved.