Infrared spectra analysis on some of A-stage benzoguanamine-formaldehyde resins with various degree of addition has been performed. The assignment of major IR absorption bands of the resins has been discussed in detail. The thermal curing behaviour of some samples is followed with the techniques of high-temperature IR and differential spectra. From the variation of IR absorption of methylol group

methylene group and dimethylene ether group of cured resins

and the measurement of formaldehyde in the gas evolved in reaction system

the reac-tions of thermal curing of the resins are discussed.