The effect of thermoreversible physical aging on the linear viscoelastic creep properties of a thermoplastic composite was investigated. Radel X/IM7, an amorphous composite material considered for use in the next generation high speed transport aircraft, was studied. The operating environment for the aircraft material will be near 188°C (370°F) with a service life in excess of 60,000 hours at temperature. Accurate predictions of the viscoelastic properties of the material are essential to insure that design strength and stiffness requirements are met for the entire service life. The effect of physical aging on the creep response was studied using momentary tensile creep tests conducted at increasing aging times following a rapid quench from above the glass transition temperature (Tg) to a sub-Tg aging temperature. As the aging time increased, the creep response of the material significantly decreased. The tensile creep compliance data for each aging time were fit to the empirical equation for the creep compliance D(t): D(t)-Dₒ<i>e</i> ^{t/tₒ)m} where Dₒ, tₒ,and m are fitting parameters determined using a nonlinear fitting program based on the Levenberg-Marquardt finite difference algorithm. The short-term creep compliance curves, obtained at various aging times, were then shifted to form a momentary master compliance curve. The double logarithmic aging shift rate μ and its dependence on sub-T_g aging temperature were determined. The aging characterization process was conducted on unidirectional specimens with 0, 90, and 45 degree fiber direction orientations. This permitted the calculation of the complete principal compliance matrix for the composite material. The effect of physical aging becomes more apparent during long-term tests when creep and aging occur simultaneously. This results in a gradual stiffening and decrease in the creep response with increased time. Predictions based solely on the Time-Temperature Superposition Principle would significantly over-predict the creep response if physical aging effects were ignored. Theoretical predictions for long-term creep compliance were made using an effective time theory and compared to long-term experimental data for each fiber orientation. Finally, experimental results of a long-term test of a 30 degree fiber angle orientation specimen were compared to theoretical predictions obtained by transforming the principal compliance matrix to the 30 degree orientation. / Ph. D.