The birefringence of polyolefins undergoes a change from an initial negative value at low elongation to a positive value at higher elongations. The extent of the negative contribution depends upon time after quenching the sample and upon temperature. The changes result from a change in the mode of crystal orientation. A model involving spherulite deformation and reorientation of crystals within the spherulite is proposed. Changes with time and temperature are ascribed to changes in a parameter &ggr; describing the mobility of crystals within the spherulites. The model is also used to account for variation in the dynamic birefringence with straining frequency.