The thermal and flame-retardant properties of phenol-formaldehyde-type resins (crosslinked and noncrosslinked) in mixtures with triphenyl phosphate and styrene-acrylonitrile resins were evaluated. The mixtures show a synergistic effect between triphenyl phosphate and novolacs. Those containing phenol-formaldehyde novolac resins are found to be most flame retardant. There does not seem to be a relationship between the oxygen index( OI) and UL 94 tests. Scanning electron microscopy analysis show a surface structure with cavities and stratification, very similar to that of intumescent additives. Evidence was found indicating that this flame-retardant system works in both the gas and condensed phase.