Transient moisture conditions can accelerate the viscoelastic behavior of certain materials over that of constant moisture conditions. This is termed the mechano-sorptive phenomenon. The thrust of this research effort is to study the mechano-sorptive effects on the creep behavior of synthetic fibers and composite materials. This study consisted of two main parts: 1). a phenomenological investigation of the transient moisture effects in synthetic fibers and composite materials and 2). mechanistic studies of the observed phenomenon. The materials studied included Kevlar® fibers, Kevlar® fiber reinforced composites, Technora fibers, poly(methyl methacrylate) (PMMA) fibers, and Nylon 6,6 fibers. Unidirectional ( 0° ) Kevlar® 49/7714 epoxy coupons undergoing desorption exhibited an increase in tensile and bending creep deformations, a decrease in storage modulus, and an increase in the loss tangent (Tan δ) when compared to coupons maintained at a constant (saturated) moisture content. However, the transient moisture effects were not seen in composite coupons along the matrix direction. Experimental results showed that aramid fibers exhibited logarithmic creep behavior under tensile load. Even though different constant moisture conditions did not have appreciable effects on the creep behavior of aramid fibers, the creep process increased substantially under transient moisture conditions. The logarithmic creep rates and the mechano-sorptive effects increased with temperature. The creep activation energies of Kevlar® fibers are: 4.84 Kcal/mole for the cyclic moisture conditions and 1.04 Kcal/mole for the constant (saturated) moisture condition. Increases in stress may increase the logarithmic creep rates but may reduce the mechano-sorptive effect. In addition, the creep behavior under transient moisture conditions was nonlinearly dependent on stress. The fiber elastic compliance was observed to increase after creep deformation. Moreover, it was found that the fiber elastic compliance has correlation with the logarithmic creep rates. Aramid fibers contain hydrogen bonds between rod-like crystallites oriented at small angles relative to the fiber axis. These hydrogen bonds may be disrupted during a transient moisture process. The breakage of these hydrogen bonds may cause slippage of hydrogen bonded crystallites and result in accelerated crystallite rotations, thus causing increases in logarithmic creep rate. Analysis indicated that the obtained activation energy (4.84 Kcal/mole) and the reduction in fiber elastic compliance due to creep deformation support the proposed mechanisms. / Ph. D.