Boron-containing novolac resins were prepared through the modification of a commercial novolac resin with different contents of bis(benzo-1,3,2-dioxaborolanyl) oxide. Their thermal and flame-retardant properties were measured. Then, they were crosslinked with hexamethylenetetramine, and their thermal, thermodynamomechanical, and flame-retardant properties were evaluated. Their modification degree was related to the segmental motion of the materials. The crosslinking of the boron-modified novolac resins with hexamethylenetetramine was slower and not as extensive as that of commercial novolac resins because the nitrogen from intermediate species coordinated with boron. The thermal degradation of the boron-containing novolac resins generated boric acid at high temperatures and gave an intumescent char that slowed the degradation and prevented it from being complete. (c) 2006 Wiley Periodicals, Inc.