Novel systems, useful in molding and in prepreg formation, contain two components capable of reacting to form an infusible material. The two components are preferably an unsaturated polyester and a crosslinker comprising a substituted acrylamide or similar compound. The systems are substantially non-volatile; solid at ambient temperatures; liquid, homogeneous and non-reactive at higher temperatures; readily and reversibly convertible from solid to liquid; and capable of conversion to said infusible material at still higher temperatures.