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 method. The poly(ethylene oxide) monomer is formed of an oxygen atom and an ethylene group. Using the random phase approximation, 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 phase behavior (whereby phase separation occurs upon heating), while the third one (oxygen/ethylene) is characterized by an upper critical solution temperature phase diagram (whereby phase separation occurs upon cooling). It is argued that clustering is caused by the increasing repulsive interaction between oxygen and ethylene for decreasing temperature and increasing polymer volume fraction. This leads to increasing attractive interactions between ethylene groups that stick together.