The morphologies of a ternary blend of nylon 66 and polystyrene in a polypropylene matrix with and without compatibilization by an ionomer resin (for nylon 66) and a styrene-block-ethylene-co-butylene-block-styrene (SEBS) copolymer (for polystyrene) were investigated by transmission electron microscopy (TEM) of stained thin sections. The morphology found with the two compatibilizers (a five-component mixture) was essentially that of the binary blends of nylon 66/polypropylene and of polystyrene/polypropylene with their respective compatibilizers, indicating no gross interference between the two compatibilization systems. However, several interactions were discerned: 1) an association of the polystyrene with the nylon in the compatibilized blends (partial wetting), 2) a presence of larger particles when both compatibilizers were added to the binary blends, and 3) a possible synergism, in which less of each compatibilizer was needed when they were both present.