The melt rheology of a new liquid crystalline copolyester Vectra E from Hoechst-Celanese Corp. was investigated. A Rheometrics mechanical spectrometer was used to characterize the change in melt rheology produced by annealing below the melting temperature with dynamic, step shear rate, and step strain tests. The coupled relaxation model was employed in analyzing the step strain test data. Typically, thermotropic liquid crystalline copolyesters exhibit a reversible annealing effect which is erased by heating above the melting transition followed by rapid cooling. The unique feature of Vectra E is an irreversible annealing phenomenon. The unannealed copolyester melts near 330 °C, above which the viscosity decreases by at least two orders of magnitude. After annealing at 300°C for 16 h, the copolyester exhibits a large increase in the melting transition temperature to 380 °C. Furthermore, upon melting the annealed sample the shear relaxation modulus and viscosity of the melt remain 2–3 orders of magnitude greater than those for the unannealed sample. These observations are consistent with a significant increase in the molecular weight of the Vectra E copolyester during annealing in the solid state at 300 °C. © 1992, The Society of Rheology. All rights reserved.