The phenolic cleavage of the uncured epoxy resins is further substantiated by the semiquantitative study with the aid of the mass spectrometer analysis. In general, the phenolic cleavage (at 475°C.) of the epoxylated novolac resin, D.E.N. 438 resin, was found to be similar to that of the unepoxylated novolac resin. For both cases, cresols and phenol were the major products as a result of the homolytic cleavage and the subsequent hydrogen abstraction. At the same time, a small amount of xanthene or the substituted xanthenes appeared in the mixture presumably due to the dehydration of related phenolic compounds. A small amount of C6H5OC3H3 or C6H4OC3H4 could indicate that there could be some cyclization of the glycidyl ether side chain of the epoxylated novolac resin. For the epoxylated bisphenol-A resin, D.E.R. 331 resin, the phenolic cleavage at high temperature appeared to follow two major and one minor scheme. The first type of cleavage is undoubtedly homolytic to yield isopropylphenol, C2H5 phenols, cresols, and phenol. The second type cleavage was not found in the above two resins and is heterolytic in nature. This cleavage resulted in the formation of isopropenyl-phenol. Due to the presence of C6H5OC3H3 or C6H4OC3H4, it is postulated that the cyclization of the glycidyl ether side chain may also, take place.