Transmission electron microscopy (TEM) was used to examine the morphologies of nylon 6/ABS blends compatibilized with a styrene/maleic anhydride (SMA) copolymer containing 25% maleic anhydride (SMA 25). Several staining techniques were employed for identifying the various phases. The morphologies of a nylon 6/ABS blend compatibilized with an imidized acrylic polymer and the commercially available Triax(TM) material were also examined by these TEM techniques. While increasing concentration of the SMA 25 copolymer clearly leads to more efficient dispersion of the ABS phase, there is an optimum level of SMA 25 to achieve maximum toughness. Various factors that might contribute to the subsequent loss in toughness with higher SMA 25 levels are discussed. It is concluded that the limitations of the SMA 25 copolymer as a compatibilizer stem mainly from its high level of reactive functionality.