The dynamic light scattering results presented in this letter demonstrate that the clustering of poly(ethylene oxide) (PEO) can be observed even in ultrapure, freshly double-distilled and filtered deionized water. It is confirmed that the filtration of solutions removes the clustering structure and that a steady-state amount of PEO in clusters is reformed in filtered solutions within 24 h. Adding a drop of chloroform to unfiltered aqueous solutions of PEO temporarily alters the clustering structure, but it prevents the clustering of PEO in filtered solutions.