Melt rheology and morphology of nylon-6/ ethylene propylene rubber (EPR) blends were studied as a function of composition, temperature, and compatibilizer loading. Uncompatibilized blends with higher nylon-6 content (N90 and N95) and rubber content (N5 and N10) had viscosities approximately intermediate between those of the component polymers. A very clear negative deviation was observed in the viscosity-composition curve over the entire shear rate range studied for blends having composition N30, N50, and N70. This was associated with the interlayer slip resulting from the high-level incompatibility between the component polymers. The lack of compatibility was confirmed by fracture surface morphology, given that the dispersed domains showed no sign of adhesion to the matrix. The phase morphology studies indicated that EPR was dispersed as spherical inclusions in the nylon matrix up to 30 wt % of its concentration. A cocontinuous morphology was observed between 30 and 50 wt % nylon and a phase inversion beyond 70 wt % nylon. Various models based on viscosity ratios were used to predict the region of phase inversion. Experiments were also carried out on in situ compatibilization using maleic anhydride-modified EPR (EPR-g-MA). In this reactive compatibilization strategy, the maleic anhydride groups of modified EPR reacted with the amino end groups of nylon. This reaction produced a graft copolymer at the blend interface, which in fact acted as the compatibilizer. The viscosity of the blend was found to increase when a few percent of modified EPR was added; at higher concentrations the viscosity leveled off, indicating a high level of interaction at the interface. Morphological investigations indicated that the size of the dispersed phase initially decreased when a few percent of the graft copolymer was added followed by a clear leveling off at higher concentration. (C) 2004 Wiley Periodicals, Inc.