Abstract: A phenol-aldehyde resole resin is disclosed which is produced in the presence of a plasticizing amount of a phenolic ester reaction product of an unsaturated fatty acid with a phenol. These compositions are characterized by improved flexibility and moisture resistance. The resins are especially useful for impregnating cellulosic materials. Particular utility has been found for these compositions as filter elements wherein the impregnated cured paper can be corrugated after curing without cracking to provide an economical and superior filter paper.