Retention performance of the polyethylene oxide (PEO)-phenolformaldehyde resin (PFR) retention aid system is affected by the chemical structure and the molecular weight distribution of PFR.1NMR and high performance liquid chromatography have been used to characterise various resole and novolak-based resins. Of the resins examined, those with the highest molecular weight gave the greatest retention improvement. The type of substitution around the aromatic ring of the resin appears to affect the interaction with PEO and the size and structure of the network formed when PFR and PEO are mixed together.