Low toughness is a major drawback with most cross-linked thermosetting materials, including the cyanate ester networks. Reactive functional thermoplastic toughness modifiers not only enhance toughness but also permit highly desirable stability to solvent stress cracking without seriously affecting the moderately high modulus. Careful control of the heterophase morphological structure is necessary to achieve significant toughening. Tn the present work, hydroxyl functional phenolphthalein-based amorphous poly(arylene ether ketone)s, poly(arylene ether sulfone)s and poly(arylene ether phosphine oxide)s were investigated as potential toughness modifiers for the bisphenol-A based cyanate ester networks. In particular, the use of poly(arylene ether sulfone)s resulted in remarkable improvements in toughness. Multiphase networks were generated without compromising either T-g or the moderately high modulus of the unmodified cyanate ester networks. It was demonstrated that the toughenability of these systems is governed by microphase-separated morphologies and the sizes of the phase separated domains. The formation of these heterophase morphological structures is strongly influenced by the backbone chemistry and the molecular weight of the thermoplastic modifier, probably via control of the polymer-polymer interaction parameter.