This article sheds light on some structural changes in thermally treated viscose fibers at a constant temperature of 100 +/- 1 degrees C at different time periods. The obtained optical parameters were used to calculate the crystallinity, density, Herman's orientation function, average orientation, and form birefringence. In addition, the stress optical coefficients, thermal stress, molar refractivity, specific refractivity, and polarizabilities along and across the axis and segment anisotropy were obtained. Measurements of the refractive indices helped us to calculate the dielectric constant, dielectric susceptibility, surface reflectivity, and transparency transmittances. The mechanisms of structural variation for the viscose fibers due to the annealing process were examined with the structural details for optothermal parameters. The relationships between the measured and calculated parameters are given in illustrations and curves. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1306-1312, 2010