An in-house developed co-rotating batch mixer was used to prepare the blends of natural rubber (NR) and ethylene-propylene-diene terpolymer (EPDM) in the present work. Phase morphology and magnitude of dispersive mixing efficiency offered by the in-house developed co-rotating batch mixer and a conventional counter-rotating batch mixer were compared. It has been found that the co-rotating batch mixer equipped with the MX2 rotor configuration could improve the dispersive mixing efficiency of NR/EPDM blends considerably. A poor state-of-mix in blends, particularly at high fill factor, could be overcome by the utilization of MX2 rotor configuration where the extensional flow is probably facilitated in the converging zones. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3688-3695, 2012