Significant progress in synthesizing and processing organic polymer molecules has led to an ever-growing materials base. These developments, along with theoretical advances, have led to increasing knowledge of structure-property relationships, especially of the role of molecular chain extension and orientation in fibres. This paper describes the molecular orientation of six polyester fibres manufactured in Egypt under different mechanical and environmental treatments. A two-beam interferometric technique is applied to determine the radial variation of the refractive indices and birefringence in these fibres.