The water-soluble complex of polyethylene oxide (PEO) with poly (vinyl phenol-co-potassium styrene sulfonate) (PVPh-co-KSS) was studied by liquid-state NMR. PEO showed two peaks in the H-1 spectra, which corresponded to the free and complexed PEG. The ratio of the free PEO/complexed PEO was decreased with the increase in the mixing ratio of PVPh-co-KSS/PEO. Some of the complex formation disappeared when the pH was raised from 6.4 to 12.0. It had been thought that at high pH, the phenolic groups dissociate and thus cannot form hydrogen bonds. The fact that NMR indicates some interaction at pH 12.0 implies there are some other interactions, such as hydrophobic interactions between the aromatic rings and the polyether methylene groups, contributing to PEO and PVPh-co-KSS complex formation. Nuclear Overhauser effect (NOE) cross peaks were observed between PEO and the aromatic protons of PVPh-co-KSS in nuclear Overhauser effect spectra (NOESY) suggesting that the distance between PEO and the aromatic protons of PVPh-co-KSS was less than 5 Angstrom. The exchange between the complexed PEO and the free PEO was slow on the NMR time scale. The ratio of the integral of the complexed PEO to the free PEO increased with temperature, indicating that the number of PEO segments interacting with the aromatic ring increases with temperature.