Rubber-modified polystyrene (PS) owes its impact resistance to a morphology of salami domains: in the PS matrix, polybutadiene (PB) domains are dispersed that, in turn, are strongly filled with PS subdomains. This unique structure is created in situ as styrene is polymerized in PB/styrene solutions. The salami domains are built up by graft copolymer chains PB-g-PS. It was suggested previously that these domains assume their characteristic architecture because strongly grafted chains stabilize them on the outside while weakly grafted chains provide their internal substructure. This model is extended in this paper that deals with the polymerisation of methylmethacrylate (NMA) in EP/MMA solutions of a poly(ethylene-co-propylene) copolymer (EP). Intermediate and final products were characterized by H-1 NMR spectroscopy, transmission electron microscopy, two-dimensional chromatography and statistical calculations, with a focus on the pivotal stage of phase inversion during the polymerisation where the salami domains are born.