The present invention relates to reactive systems comprising(a) a storable mixture of (i) a polyisocyanate component comprising at least one organic polyisocyanate, and (ii) an epoxide component stabilized by heating in the presence of an alkylating agent, wherein said heating is optionally carried out in the further presence of part of the polyisocyanate component (i),(b) an isocyanate-reactive component containing at least two isocyanate-reactive groups in a quantity such that 0.1 to 0.9 isocyanate-reactive groups are present for every isocyanate group in component (a), wherein at least 10% by weight of component (b) comprises compounds having a molecular weight greater than about 2000,(c) optionally, tertiary amines in a quantity such that the total storable mixture contains at least 0.005 mole of tertiary amine nitrogen atoms for each 100 g of the total amount of the storable mixture, and, optionally,(d) other auxiliaries and additives.The present invention also relates to a process for the preparation of optionally foamed polyurethane plastics containing isocyanurate groups comprising (1) mixing the components of such reactive systems at a temperature less than 60.degree. C. and (2) heating said reactive system to a temperature of at least 60.degree. C.