SynopsisSeveral solvent systems are presently used to characterize the molecular weight of phenol-formaldehyde resins. However, results reported in the literature for different molecular weights may not be representative of true molecular weight, but rather may be distorted by aggregation and solvation. In this report an effort to clarify this situation was conducted, first by using a suitable calibration with poly(ethy1ene glycol) and then using this polymer as a molecular size standard to determine the size of phenol-formaldehyde oligomers in solution. In the calculation of the molecular sizes of phenol-formaldehyde resols, proper accounting of the variation of the Mark-Houwink parameters with molecular weight for low degrees of polymerization must be made for poly(ethy1ene glycol). The Mark-Houwink constants for poly(ethy1ene glycol) are very similar in presence or absence of salts, and are considered to be unaffected by solution ionic strength. It is not the case for phenol-formaldehyde, w h a e apparent molecular size varies with the nature of the solvent. The actual molecular weight and molecular dimension distribution are discussed for different type A resols used as adhesives in the wood composite industry.