Abstract: This invention is a process for preparing polyurethane and/or polyurea elastomer comprising reacting a liquid, preformed isocyanate-terminated prepolymer, which prepolymer is the reaction product of a stoichiometric excess of a polyisocyanate and a relatively high equivalent weight isocyanate-reactive compound, with a chain extender mixture, which chain extender mixture comprises(a) a mixture comprising a hydroxyl-terminated chain extender and an aromatic amine terminated chain extender, and/or an aliphatic amine-terminated compound, or(b) a mixture comprising an aromatic amine-terminated chain extender and an aliphatic amine-terminated compound, wherein the isocyanate index is from about 70 to about 200.In this process, it is possible to prepare elastomers having excellent properties, particularly excellent flexural and thermal properties, which properties are difficult to obtain in a one-step process.