Blends of ethylene-propylene rubber (EPM) and nylon 6 are immiscible and highly incompatible. These blends are characterised by a two-phase morphology, narrow interface, and poor physical and chemical interactions across the phase boundaries. Therefore a reactive route was employed to compatibilise these blends by the addition of maleic anhydride grafted EPM (EPM-g-MA). In this reactive route, the maleic anhydride group of EPM reacts with the amino end group of nylon forming a graft copolymer of nylon and EPM (nylon-g-EPM) at the blend interface which decreases the interfacial tension and reduces the coalescence. The influence of the concentration of EPM-g-MA, blend composition, molecular weight of nylon 6, mode of addition of EPM-g-MA and mixing time on the phase morphology of the blends was studied quantitatively by scanning electron microscopy and image analysis. It was found that the addition of EPM-g-MA reduces the domain size of the dispersed phase followed by a levelling off at high concentrations; the levelling off is an indication of interfacial saturation. The optimum amount of the compatibiliser required to saturate unit volume of the interfacial zone was estimated from the emulsification curves. This optimum concentration of the compatibiliser can be considered as the so-called critical micelle concentration (CMC) above which micelles of the copolymer are formed in the bulk phase, which is highly undesirable. Emulsification master curves were obtained by plotting the reduced domain size as a function of EPM-g-MA for the different blend compositions. This was explained based on the interfacial area occupied by the compatibiliser molecule at the blend interface. The experimental results were compared with the current compatibilisation theories. The phase morphology development was studied as a function of the mixing time in the presence and absence of compatibiliser. The influence of reactive compatibilisation on phase inversion and the co-continuous nature of the blends was also investigated. Finally the stability of the blend morphology was analysed by high temperature isothermal annealing in the presence and absence of compatibiliser (C) 1999 Elsevier Science Ltd. All rights reserved.