Volume relaxation in polymers and the effect intrinsic to glassy polymers can significantly affect their refractive index over time. Its beta rate has been found to be related only to relaxation temperature T and the glass transition temperature of the polymer Tg and not to the polymeric chemical structure. Universal values of beta have been obtained for polymers and were used to predict the minimum index change related to volume in polymers. The index change is in the range from 7.86 x 10(-5) to 5.26 x 10(-4) when the Tg - T value of polymers is between 90 and 350 degrees C. These volume-relaxation-induced changes can cause serious deterioration or even failure in corresponding polymer waveguide devices, such as arrayed waveguide gratings and variable optical attenuators, when the Tg of a polymer is not sufficiently high. A minimum requirement is therefore suggested for the Tg of polymers used to fabricate waveguide devices.