The presence of moisture in a polymer often accelerates the creep process, presumably through a plasticization effect. Previous studies on wool, wood, and wood products have indicated that the creep and dynamic loss can be significantly increased for conditions of changing moisture content. Our recent tests have indicated that similar trends occur in certain man-made fibers and their composites. This suggests that not only the moisture content, but also the sorption history are important for determining the viscoelastic properties of polymeric fibers and composites. We have previously shown that the logarithmic creep rate of Kevlar^® 29, 49, and 149 fibers subjected to 2-h wet/dry cycles increases by as much as a factor of 3 over the creep occurring under constant, saturated moisture conditions. In the current paper, we will show that similar increases in creep rates are exhibited by other aramid based fibers: Teijin's Technora^® and Akzo's Twaron^® products. We have also extended the investigation to other materials including polyethylene, poly (methyl methacrylate), and cellulose acetate butyrate fibers. The latter fibers were spun in our own laboratories, allowing us to evaluate the effects of spinning rate on the mechano-sorptive behavior. We were also able to dope the fibers with lithium chloride (LiCl) in order to assess the effect of a salt on the mechano-sorptive behavior. It had been suggested by other researchers that the presence of a contaminant might increase the susceptibility to transient moisture conditions because of osmotic action. To the contrary, we actually found that the presence of LiCl tends to decrease the effects of transient moisture conditions. Further comments are made concerning the suspected mechanisms that are responsible for the mechano-sorptive phenomena in these material systems.