Rheological techniques, like oscillating rheometry and high-pressure capillary rheometry were used to measure and to describe chemical and physical aging influences on the rheology of polyamide and polybutylene terephthalate in an advanced viscosity model. Viscosity drop was found as cause of combined time and temperature effects and understood as chemical aging that was reduced to a thermally induced degradation. The influence of chemical aging on viscosity was described based on a novel structural change shift factor a(SC). In addition, viscosity drop was found as cause of residual moisture. Its influence was described by a novel physical change shift factor a(PC). The functional relationships for both factors were derived from measured data. Moreover, both factors contain additional parameters that were optimized with respect to specific processing and boundary conditions. The consideration of chemical and physical aging by means of a(SC) and a(PC) provides a better understanding with respect to rheological changes and thus a better control of process variations and an enhanced product quality. POLYM. ENG. SCI., 55:1628-1633, 2015. (c) 2014 Society of Plastics Engineers