Milling behavior of brominated isobutylene-co-paramethylstyrene (BIMS), and its blends with ethylene propylene diene rubber (EPDM), with special reference to the effect of addition of different fillers, filler loadings and processing oil, was investigated over a range of temperatures in a drop mill operation. The critical nip gap (CNG), at which a front to back roll transition (F-B transition) occurs, was also measured. Addition of different fillers reduced the value of the critical nip gap and the mill band formation indices for BIMS as compared to the gum polymer, indicating a decrease in the tendency to go to the back roll. The systems containing reinforcing fillers showed a lower value of the critical nip gap, as compared to systems containing low or non-reinforcing fillers. For different filled blends of BIMS and EPDM, the CNG decreased on increasing the EPDM content, indicating a decrease in the tendency for a F-B transition. The CNG decreased on increasing the N330 carbon black-loading, while it increased with the addition of processing oil. The results were explained in terms of rubber to metal adhesion and viscosity of the polymers.