AbstractThis paper reports on the study of the mechanical and dynamic properties of composites with an EPDM matrix and short fiber, either polyester or carbon, at three different fiber concentrations. In general, the properties prove to be dependent on fiber concentrations and type, in particular on the final ratio fiber length/diameter. This ratio ranges for carbon fiber over 35–45 and for polyester over 135–175, thus placing the former near the lower limit of acceptability. Evidence is supplied of the existence of adhesion between the matrix and the fiber, in the form of measurements of the swelling and dynamic properties, especially through variation of relative damping and the displacement of the dynamic glass transition temperature towards higher ranges. Composites present a marked property anisotropy.