Oligomers comprising four or sixteen phenylalanine residues with regularly intercalated aliphatic chains of different lengths prepared by solid-phase synthesis exhibit sufficient thermal stability to be used as interfacial agents and processed for the preparation of poly(propylene-co-ethylene)-based composite materials. The investigation of their adsorption on Kevlar fibers by SEM is difficult due to the surface heterogeneity of the bare Kevlar fibers. However, oligomers with four successive phenylalanine residues have been clearly observed suggesting their better adsorption on the fiber. The quantification of those oligomers adsorbed on the fibers performed gravimetrically on pellets of fibers has however revealed no significant impact of the length of the aliphatic chain.