the determination of rotational correlation times for spin probes in polymers is reviewed, with emphasis on the relationships between rotational and translational motions. data on the behavior of three different spin-probe molecules in polyamide and polyester films and fibers are examined in detail. it is concluded that saturation phenomena make it difficult to obtain reliable rigid limit parameters in these systems, unless very low microwave power levels can be used. at very low temperatures probe rotation appears to take place in local defects or microvoids in the glassy polymers there is an increase in activation energy of rotation at a temperature close to the t/sub g/ expected from dynamic mechanical (110 hz) measurements, but the probe molecule rotations remain strongly hindered until significantly higher temperatures are reached. in nylons, sensitivity of probe rotations to structural differences between fibers is low in dry systems, and increases with temperature and moisture content. even in nylon fibers containing substantial amounts of water, the probe rotation is most consistent with free-volume concepts, and is inconsistent with models of probe rotation and diffusion in solvent-filled pores or channels