Method and apparatus for molding polymeric materials using the reaction injection mixing (RIM) technique of combining two or more chemically reactive components immediately prior to their introduction into a mold. The invention is more particularly adapted to RIM molding of polyamide (nylon) type polymers, but is also useful for urethane polymers. In either case the polymers may be of the unfilled or of the reinforced type. Modification of conventional RIM technique is made to meet higher component temperature conditions required for nylon molding as compared to urethane polymers, for example. A characterizing feature of the invention is the employment of an attempering "oil tracer" system comprising casings and jackets enclosing the flow paths of each of the several reactive components, and the flow therethrough of a temperature-controlled oil or other fluid medium in heat-exchange but non-fluid communication with the reactive components. Arrangement is made for zoning the component flow paths, and selectively controlling the delivery of attempering oil to different zones whereby to provide flexibility of temperature control at different zones.