This is a continuation of work on interactions between polyamide 66 (PA66) fibers and CaCl2/H2O/EtOH mixtures. It was observed that the mixtures dissolved the fibers, but with or without an intermediate stage of visibly evident swelling depending on the mixture composition. The interaction proceeds via Lewis acid-base complexation between the polymer carbonyl groups and Ca2+ ions and can be interrupted by rinsing the fibers with water. Swollen fibers retained their expanded diameters even after rinsing and exhibited a highly rough surface and increased water retention. The observed effects suggest that such mixtures may be used to increase the surface roughness of PA66 fibers for increasing the interfacial adhesion in composites applications. In this publication, we report the results of further investigations into the spatial structure of cross sections of swollen fibers. Using atomic force microscopy coupled with infrared spectroscopy on the length scale of 100 nm (nanolR-AFM), we could show, for the first time, the PA66 core-shell structure, where the shell thickness increases with the treatment extent and exhibits a highly porous structure. Thus, the surface roughness observed previously is not limited only to the surface but extends toward the fiber core. The examination also showed no evidence of Ca2+ complexation in the fiber cores, which confirms a near-complete removal of the ions. Additional measurements of the crystallinity with differential scanning calorimetry and attenuated total reflectance Fourier transform infrared spectroscopy showed that the shell exhibits lower crystallinity than the core.