In this work the physical properties oi composite materials consisting of an elastomeric EPDM matrix and short polyester fibre (PET) are examined as a function of fibre content and preferential fibre alignment The PET fibre was used in its untreated form and treated with a diazide derivative, which acts as a crosslinking bridge between the matrix and the fibre. Variation of different properties, such as Shore hardness A, rebound resilience, compression set abrasion and tensile properties - stress at constant strain, as well as tensile strength, elongation at break, and tear strength -, are interpreted in terms of the existence or absence of a matrix-fibre interface generated through surface modification of the fibre.