Results are presented for the temperature dependence of orientation birefringence, &Dgr;n(T), from −195° to 150°C for the following crosslinked polymers: polydimethyl siloxane, polyethylene, ethylene/propylene copolymers (84/16, 75/25, 48/52 mole ratio), and ethylene/vinyl acetate copolymers (96/4, 82/18). On cooling from the rubbery state under a small constant load, the birefringence magnitude shows an abrupt increase when crystallization takes place, reaching a maximum value (&Dgr;nc) when crystallization is complete; &Dgr;ncis determined principally by the degree of orientation in the rubbery state. In the rigid semicrystalline state, belowTg, the magnitude of orientation birefringence decreases with decreasing temperature. The transition temperature for the ethylene/propylene copolymers is lowered by increasing the propylene content; the nature of the transition changes progressively from a crystallization in polyethylene to a glass transition in the copolymer having 52 mole % propylene. The observed difference in optical parameters for polyethylene and its copolymers suggests a difference in physical structure which strongly affects the optical properties. We believe that the structure of the copolymers is microcrystalline, contrasting to the spherulitic crystallinity of polyethylene (and polydimethyl siloxane).