This work involves the development of novel glass fiber-reinforced composite materials containing a commercially available epoxy resin, a phosphate-based intumescent, and inherently flame-retardant cellulosic (Visil, Sateri) and phenol-formaldehyde (Kynol) fibers. The intumescent and flame-retardant fiber components were added both as additives in pulverized form and fiber interdispersed with the intumescent as a fabric scrim for partial replacement of glass fiber. Thermal stability, char formation, and flammability properties of these novel structures were studied by thermal analysis, limiting oxygen index, and cone calorimetry. The results are discussed in terms of effect of individual additive component on thermal degradation/ burning behavior of neat resin. (C) 2003 Wiley Periodicals, Inc.