Polymers blends are usually immiscible due to their small combinatorial entropy of mixing. Miscibility can be achieved by promoting intermolecular exothermic interactions such as hydrogen bonding, ion-dipole interactions, acid-base interactions, and transition metal complexation. An increasingly popular strategy for enhancing the miscibility of two polymers is to exploit the interactions between the ionic species of an ionomer and a complimentary functional group on a second polymer. An alternative approach for achieving miscibility in a polymer blend is to exploit the “copolymer effect”, whereby strong repulsive interactions along one polymer chain promote mixing of that polymer with another polymer, even when the binary interactions between all distinct chemical species are unfavorable. This strategy of achieving miscibility is predicted from the Binary Interaction Model, proposed by Paul and Barlow. In this talk, we discuss why ionomers represent an extremely versatile class of compatibilizer. That is, ionomers can mix with a wide variety of other polymers as a result of either specific attractive intermolecular interactions or strong intramolecular repulsive interaction within the ionomer. In particular, we will discuss the remarkable control of the phase behavior of sulfonated polystyrene/polyamide, sulfonated polystyrene/polycarbonate blends and poly(acrylic acid)/poly(ethylene oxide) blends.