The processing behavior and morphology are investigated for nonreactive and reactive nylon/ethylene-propylene, rubber blends. The nylon (PA) is a partially aromatic amorphous nylon with amine termination. The rubbery phases are an ethylene-propylene rubber (EP) and a similar ethylene-propylene rubber (EP-MA) with 0.7% grafted maleic anhydride. The nonreactive PA/EP blends show poor interfacial adhesion between the two phases. The reactive PA/EP-MA blends show excellent adhesion and much smaller dispersed phase domain sizes. The interfacial chemical reaction affects several separate parameters which influence the size of the dispersed phase: the formation of a copolymer at the interface which reduces the interfacial tension; different rheological properties of the modified and unmodified rubbers; the effect of the interfacial reaction on the rheology of the blend; changes in process parameters die to the effect of the interfacial reaction; reduction in the rate of particle-particle coalescence due to the presence of the copolymer at the interface; as well as other effects. A semi-empirical relationship is used to separate the effects of phase rheology and applied mixing stress.