Thermoelastic. networks were prepared by end-linking hdroxyl-terminated poly(neopentylglycol hexafluoroglutarate) using tris(p-isocyanate phenyl) thiophosphate as cross-linking agent. The plot of birefringence versus stress exhibits negative deviations from linearity at elongation ratios larger than 1.9. The value of the optical configuration parameter Deltaa is 2.98 Angstrom (3) at 5 degreesC. Theoretical calculations carried out using the rotational isomeric state model give far this parameter the value of 2.24 Angstrom (3), in fair agreement with the experimental results. The cause of the strong discrepancies between the theoretical. and experimental results observed for the optical configuration parameter of polyesters containing phenyl groups in the acid residue is discussed.