Binary suspensions made up of oppositely charged nanoparticles have been shown to self assemble into substitutionally ordered superlattices. The structure of the superlattice depends upon the particle charges as well as the mixture composition. Pressure composition phase diagrams for binary mixtures of equal sized and oppositely charged nanoparticles are constructed for charge ratios ranging from -1 to 0. The effect of charge asymmetry on the formation and stability of the substitutionally ordered solid is clearly demonstrated.