Clustering prevails in water-soluble polymers and biological macromolecules. It has also been observed in polar solvent mixtures. The possible causes of clustering are discussed. A systematic investigation of clustering in poly(ethylene oxide)/d-water solutions has been undertaken using the small-angle neutron scattering (SANS) method. The PEO monomer is formed of an oxygen atom and an ethylene group. Using the Random Phase Approximation (RPA), partial Flory-Huggins interaction parameters for the three pairs (oxygen/d-water, ethylene/d-water and oxygen/ethylene) are derived. Results show that the first two (oxygen/d-water and ethylene/d-water) are characterized by a lower critical solution temperature (LCST) phase behavior (whereby phase separation occurs upon heating), while the third one (oxygen/ethylene) is characterized by an upper critical solution temperature (UCST) phase diagram (whereby phase separation occurs upon cooling). Clustering is caused by the tendency of this last pair (oxygen/ethylene) to demix while the polymer itself dissolves well.