A method of preparing thickened compositions which can be molded under low pressures and temperatures to provide thermoset articles, wherein liquid reactive components are slowly absorbed in solid thermoplastic elastomer thickeners such as thermoplastic ionomers and acrylic block copolymers. The thickener delays the viscosity build, allowing fiber reinforcement and other fillers to be incorporated, if desired, before molding. The molding composition is well suited for use in dentistry and other fields where small amounts or molding composition are used occasionally.