In an effort to prepare a novel novolac phenol (NP) based char former with good solubility, the hydroxyl functionalities of NP were blocked with phenyl isocyanate (PI) via a simple urethane-forming reaction. The chemical structure and properties of the obtained novolac phenol-phenyl isocyanate adduct (NP-PI) were characterized with gel permeation chromatography, Fourier transform infrared spectroscopy, H-1-NMR, and differential scanning calorimetry. Adducts of two kinds of NPs (molecular weights = 450 and 800) with PI were used as potential char formers for this study. Thus, a two-component system using NP-PI as a char former and triphenyl phosphate (TPP) as a phosphorous-based flame retardant was blended with ABS, and the thermal degradation behavior and flame retardance were examined as a function of the molar mass of NP-PI and the TPP/NP-PI ratio with thermogravimetric analysis and limiting oxygen index (LOI) testing. ABS compositions with no NP-PI were also prepared for comparison. The mixtures showed a synergistic effect between TPP and NP-PI on the flame-retardance enhancement of ABS. Those containing the higher molar mass NP-PI adduct were the most flame retardant, and a LOI value as high as 41 was obtained. (c) 2006 Wiley Periodicals, Inc.