Polyoxazolidone elastomers are obtained by reacting at 150°C. bisphenol-A-diglycidylether and polyoxytetramethylene-glycol having terminal isocyanate groups in the presence of new diepoxide-soluble catalysts. Among these catalysts, MgCl2/HMPA2 yields an excellent polyoxazolidone elastomer having low modulus and high tensile strength. The stress-strain data of the elastomers show stress-induced crystallisation. The reaction proceeds selectively and completely in one hour at 150°C. The pot life is long enough to ease the handling before curing. The properties of polyoxazolidone elastomer, especially heat resistant properties, are compared to those of polyetherurethane, also a castable elastomer. This comparison has shown (1) a somewhat higher melting point, (2) appreciably better retention of tensile properties after aging at 170°C (3) lower, high-temperature tensile properties and (4) tear energy, stress-relaxation and the creep behavior about the same or somewhat lower.