To improve the brittleness of high ortho novolac resins, at first, high ortho novolac resins (N) were synthesized and condensation reactions between N and flexible diacids such as adipic acid (DA4) and azelaic acid (DA7) were carried out with different amounts of diacids, in the presence of zinc acetate as catalyst, in toluene as solvent. To characterize the diacid-modified resins (MN), FTIR analysis, TGA and DSC techniques were used. The morphologies of MN were also investigated. Moldings were prepared from the mixture of MN, fillers, and curing agent hexamethylenetetramine. Mechanical properties of these moldings were also determined. The results showed that high ortho novolac resin was toughened by the incorporation of flexible diacids in the polymer backbone and these modified resins can be used as a matrix resin of composite materials. (c) 2007 Wiley Periodicals, Inc.