Polarized Raman spectroscopy and birefringence measurements on a wide range of as-spun poly(ethylene terephthalate (PET) and poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) fibres have been correlated. Evidence is provided for the linear relationship between birefringence and molecular orientation. An intrinsic birefringence value of 0.244 is obtained for PET and a value of 0.487 for PEN.