The effect of uniaxial and biaxial film-processing parameters on the orientation of the phenyl ring in the crystalline and amorphous phases has been evaluated using a variety of techniques. It has been demonstrated that the total phenyl ring orientation factor (PROF) measured from refractive indices can reflect the nature of amorphous and crystalline phases as well as morphological changes taking place due to variation in processing conditions. The PROF parameter can, therefore, offer a useful relationship to predict oxygen perme-ability for biaxially oriented PET films prepared under a variety of processing conditions. The cause of nonlinearity of oxygen permeability with birefringence, percent crystallinity, and tan δ maximum peak temperature (MPT) is discussed in detail. © 1993 John Wiley & Sons, Inc.