Boron-containing novolac resins were synthesized by the modification of a commercial novolac resin with different contents of bis(benzo-1,3,2-dioxaborolanyl)oxide. These novolac resins were crosslinked with diglycidyl ether of bisphenol A (DGEBA), and their thermal, thermodynamomechanical, and flame-retardant properties were evaluated. The boron-containing novolac resins were less thermally stable than the unmodified novolac resin. Their modification degree and DGEBA content were related to the crosslinking density of the materials. The boron-containing novolac resins generated boric acid at high temperatures and gave an intumescent char that slowed down the degradation and prevented it from being total. They also showed good flame-retardant properties. (c) 2006 Wiley Periodicals, Inc.