The hydrophilic nature of natural fibers and the hydrophobic behavior of standard polymers such as polypropylene and other polyolefins are the major causes for the lack of adhesion between fiber and matrix in natural fiber reinforced plastics (NFRP’s). To overcome this incompatibility of fiber and matrix, different coupling agents can be incorporated into the compound. Investigations at the Institute for Polymer Materials and Plastics Engineering (PuK) at Clausthal University of Technology revealed the bonding capability of three dendritic polyesters and one thermoplastic polyurethane in a flax fiber/PP compound. The treatment with different amounts of coupling agent led to an increase in mechanical properties such as bending strength, tensile strength and most of all CHARPY impact strength. In addition it was possible to make a correlation between IR spectra and the chemical and physical interactions between fiber, matrix and adhesive. Therefore a relationship between intermolecular forces and mechanical properties could be concluded.