The phase behavior of a series of blends obtained from mixing carboxylated poly(phenylene oxide) with sulfonated polystyrene and their respective neutralized ionomers was studied by differential scanning calorimetry. A substantially broader range of miscibility was observed when both blend components were functionalized, compared with blends in which only one of the components contained an acid group or was an ionomer. The properties of metallic cations which were used to neutralize the acid groups in the blends were found to have an effect on the miscibility. The miscibility of the two acid polymers or their ionomers depended on the difference of their functionalization level instead of on the absolute percentage of functional groups on each polymer. It was found that the two acid polymers or their ionomers remained miscible as long as they had a similar percentage of functional groups.