Abstract: Polyurethanes having improved properties are obtained from polyester-polyether co-prepolymers. These co-prepolymers are homogeneous, clear liquids of varying viscosity and can be reacted with curing agents to yield clear homogeneous elastomers of various hardness. Cast elastomers prepared from such co-prepolymers have physical properties at least as good as premium polyester urethanes currently available, and have hydrolytic stability and low temperature properties better than such premium polyester urethanes and comparable to premium polyether urethanes, with the cost being comparable to the lower cost of the conventional polyester systems.