Reactive functional thermoplastic poly(arylene ether) toughness modifiers were demonstrated to enhance toughness of brittle thermosetting cyanate ester networks and also allowed retention of a highly desirable stability to solvent stress cracking and a moderately high modulus. Careful control of the heterophase morphological structure was necessary to achieve significant toughening. In contrast to the well-defined morphologies of the reactive thermoplastic-modified networks, the use of nonreactive simple physical blend modifiers of the same molecular weight and backbone chemistry produced a macrophase separation and no apparent control over the sizes of the phase-separated domains. Macrophase-separated morphologies are inherently process-sensitive and less desirable from the point of performance control and prediction. Generation of controlled microphase-separated morphologies can be achieved by systematically varying thermal cure cycles in the case of the reactive thermoplastic-modified systems. Such a cure cycle dependence of the morphology was particularly demonstrated for the case of the 25 wt % 15,000 (M(n)) (15K) phenolphthalein-based hydroxy-functionalized poly(arylene ether sulfone) (PPH-PSF-OH)-modified networks. Morphologies that exhibit finer textures of the phase separated domains usually result in lower fracture toughness values.