Soluble polyarylates were prepared from the reaction of 2,2‐bis(4‐hydroxyphenyl)propane (bisphenol‐A) and aromatic acid dichlorides (phthaloyl chloride and related diacid dichlorides), and used to improve the brittleness of a cyanate ester resin. The polyarylates include poly[2,2‐di(4‐phenylene)propane phthalate] (PPA), poly[2,2‐di(4‐phenylene)propane phthalate‐co‐2,2‐di(4‐phenylene)propane isophthalate] (IPPA) and poly[2,2‐di(4‐phenylene)propane phthalate‐co‐2,2‐di(4‐phenylene)propane terephthalate] (TPPA). Furthermore, a commercial polyarylate, U‐polymer, was also used as a modifier. The morphologies of the modified resins depended on the polyarylate structure and concentration. The most effective modification of the cyanate ester resin could be attained because of the co‐continuous phase structure of the modified resin: 25 wt% inclusion of IPPA (50 mol% isophthalate units, weight average molecular weight (Mw) 38 500 g mol−1) led to a 130% increase in the fracture toughness (KIC) for the modified resin, with retention of its flexural properties and glass transition temperature, as compared with the values for the unmodified resin. Water absorptivity of the IPPA‐modified resin was smaller than that of the unmodified resin. Copyright © 2003 Society of Chemical Industry