The characterization of KEVLAR-29 fibers has been performed by studying their interactions with three model compounds in gaseous form. Water, aniline, and epoxystyrene were used for this purpose. The effects on the nature of these interactions of two types of surface engineering, namely the coating with an epoxy resin and a reactive plasma treatment, were also studied. The results obtained from the three analytical techniques used in this work provide a more fundamental understanding of the fiber bulk properties vs. their surface characteristics. The diffusion, condensation, plasticization, and adsorption mechanisms involved during the interactions between the probes and the KEVLAR fibers are evidenced and discussed.