Toughening of epoxy and polyimide matrices with fluorocarbon elastomers was investigated. The introduction of polar groups into the fluorocarbon backbone gave rise to compatible blends of elastomer and partially cured epoxy matrix. During postcure, however, the elastomer phase separated out. Through a careful control of postcure conditions, production of 'tailor-made' morphology and, consequently, physical-mechanical properties may be possible. In epoxy blends, a bimodal distribution of the elastomer particle size and a concomitant fourfold increase in the critical strain energy release rate were found. Blends containing the polyimide matrix were incompatible at all stages of cure. The effect of addition of elastomer on thermal stability of the blend was more pronounced with the polyimide than the epoxy matrix.