The rheology/morphology relationship of plasticized and nonplasticized ethylene-propylene-diene-terpolymer/polypropylene (EPDM/PP) TPOs was studied. The aim was to investigate the effect of a plasticizer on the morphology, specially on the co-continuity interval of these blends. The addition of a plasticizer increased the interconnectivity of the elastomeric phase, resulting in a rapid percolation of the EPDM at a relatively low composition range as compared to the nonplasticized counterparts. However, the addition of the plasticizer did not change the onset of the co-continuity interval in the low EPDM content side of the composition diagram. Moreover, due to plasticization, the percolation of the PP phase was delayed on the other side of the composition diagram. Large differences between the viscous and elastic properties of the constituent polymers were observed. Hence, a combination of low frequency measurements and a gel approach were crucial to characterize the co-continuity interval using rheology. The phase inversion compositions were fairly well described by existing semi-empirical viscosity ratio-based models. Furthermore, a satisfactory prediction was obtained for the viscoelastic properties of the nonplasticized TPOs using a micro-mechanical model. However, this model failed in the case of the plasticized TPOs, due to the probable presence of a plasticized interphase. POLYM. ENG. SCI., 51:2314-2327, 2011. (C) 2011 Society of Plastics Engineers