The chemical change of four resole type phenol-formaldehyde resins in curing process was studied by infra-red spectroscopy. It was shown that the curing process of resoles

at first

proceeded by condensation reaction of methylol groups

and the formation of dimethylene ether linkage was predominant. During further curing process (150-200°) carbonyl group appeared. The formation of carbonyl group was not caused by the ixidation of methylene group

but was a result of splitting of dimethylene ether linkage. The mechanism of suring process of phenolic resole resins was discussed.