The diffusion properties of complexes formed between poly(ethylene oxide) (PEO) and poly(vinylphenol-co-styrenesulfonate) (PKS) in aqueous medium were investigated by diffusion ordered NMR (DOSY), fluorescence photobleaching recovery (FPR), and viscosity measurements. All three techniques showed that PEO/PKS complexes range from single PEO coils with bound PKS molecules to large complex species containing many PEO chains. For a given PKS structure, there are two important transitional PEO molecular weights. The lowest one, similar to8000 Da, corresponds to the onset of PEO/PKS complex formation. The second transitional PEO molecular weight is between 10(5) and 10(6) Da and corresponds to the onset of multi-PEO chain complex species which are important for flocculation. PKS functions as a physical cross-linking agent for PEO. If there is too little PKS, multiple PEO chains are not bound together. Similarly, high concentrations of PKS give small complexes because there are few opportunities fur connecting multiple PEO chains together, since all the PEO chains are saturated with bound PKS.