Soluble polyimides (PIs) were prepared as random or multiblock types with 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) as acid dianhydride components and 4,4'-bis(m-aminophenoxy) diphenyl sulfone (m-BAPS) as a diamine component by a one-pot process and used to improve the brittleness of the cyanate ester resin. Random-type PIs were more effective as modifiers than multiblock-type PIs. The morphologies of the modified resins depended on PI structure, molecular weight, and concentration. The most effective modification of the cyanate ester resin was attained because of a heterogeneous phase structure composed of a flat matrix phase and phase-inverted structures of the modified resin; a 15 wt% inclusion of a random PI (weight-average molecular weight = 63,400) composed of 6FDA, s-BPDA, and m-BAPS (0.5/0.5/1.0 molar ratio) led to a 65% increase in the fracture toughness for the modified resin with a slight loss of flexural strength and a retention of flexural modulus and glass-transition temperature, compared with the values for the unmodified resin. Water absorptivity of the modified resin was comparable to that of the unmodified resin up to 400 h, and then, water absorption of the modified resins increased considerably. (C) 2003 Wiley Periodicals, Inc.