Abstract: Mixtures of reactive thermosetting resins which are stable at ambient temperature, latent catalyst complexes for hardening said mixtures, processes for their manufacture, and heat-hardened resins obtained from said mixtures. Mixtures of heat-reactive thermosetting resins which are stable at ambient temperature for use in impregnating insulation for electrical materials or in the preparation of molding products or composite materials, comprising liquid oligo- or poly-isocyanate compounds and liquid epoxy resins and a latent hardening catalyst which is inert at ambient temperature and is composed of an addition complex of an amine compound from the group consisting of the amines, the alkylhydrazines and the imidazoles, and a boron halide. They additionally comprise a small quantity of maleic anhydride or the boron halide is boron tribromide with no addition of maleic anhydride. Latent catalyst complexes of these mixtures, formed by the addition complexes of BBr.sub.