The present invention is directed to a process for the production of optionally cellular, polyurethane elastomer molding by reacting a reaction mixture containing(I) a polyisocyanate,(II) an isocyanate-reactive polymer having a molecular weight of about 1800 to 12,000,(III) about 5 to 50% by weight, based on the weight of component (II) of a chain extender comprising a sterically hindered aromatic diamine and(IV) an internal mold release agent mixture comprising(a) about 0.5 to 10% by weight, based on the weight of components (II) and (III) of a zinc carboxylate containing 8 to 24 carbon atoms per carboxylate group and(b) a compatibilizer comprising a member selected from the group consisting of nitrogen-containing, isocyanate-reactive, acyclic compounds and nitrogen-containing, isocyanate-reactive polymers in an amount sufficient to solubilize the zinc carboxylate so that when the internal mold release agent mixture is in admixture with components (II) and (III), the zinc carboxylate possesses improved resistance to precipitation,the reaction mixture being processed as a one-shot system by the RIM process at an isocyanate index of about 70 to 130.