Among short fiber-reinforced composites, those with rubber matrices have gained great importance due to the advantages they have in processing and low cost, coupled with high strength. These composites combine the elastic behavior of rubbers with strength and stiffness of fibers. Reinforcement with short fibers offers attractive features such as design flexibility, high modulus, tear strength, etc. The degree of reinforcement depends upon many factors such as: the nature of the rubber matrix, the type of fiber, the concentration and orientation of the fibers, the fiber to rubber adhesion and fiber length. One of the main features related to this class of composites is their hysteretic properties which are of great importance especially in dynamic applications such as tire treads. In this research, short aramid fibers with different kinds of surface treatments: Standard finish and resorcinol formaldehyde latex (RFL)-coating, have been applied in two different carbon black filled rubber compounds based on natural rubber (NR) and ethylene propylene diene rubber (EPDM). The dynamic properties of the compounds have been measured as influenced by the different kinds of fiber treatments on rubber-fiber interaction. Finally, the contribution of the interfaces and/or interphase layers to the loss properties is characterized. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013